...en el súbito iluminarse del espacio

[Quaeso]

Platero, cada vez que sonaba un estallido, se estremecía, azul, morado, rojo en el súbito iluminarse del espacio; y en la claridad vacilante, que agrandaba y encogía su sombra sobre el cabezo, yo veía sus grandes ojos negros que me miraban asustados. - Platero y yo, No. 76, Los Fuegos

I take Platero to be nominative here instead of vocative (simply "Platero trembled" instead of "O Platero...") and that the adjectives azul, morado, and rojo modify estallido. But the main question is about iluminarse, which looks to be a masculine noun, but with a reflexive suffix, se. I didn't know that a noun could be reflexive. "Platero trembled each time that a blue, purple, or red explosion sounded in the sudden self-illumination of the space"?

[Rusty]

Platero is the subject of the sentence. It was he who shuddered.
Yes, the colors mentioned go with each explosion.

A Spanish infinitive, whether reflexive or not, can act as a noun.
Its translation in this case is 'illumination' or 'lighting up' (the latter being a truer translation of the English gerund, which always acts as a noun).

[AngelicaDeAlquezar]

Adding to what Rusty said:

Your conclusion is right. Platero got scared each time the fireworks exploded and lit the sky.

<<Platero, cada vez que sonaba un estallido, se estremecía, azul, morado, rojo en el súbito iluminarse del espacio; y en la claridad vacilante que agrandaba y encogía su sombra sobre el cabezo, yo veía sus grandes ojos negros que me miraban asustados.>>

The sentence in blue is interrupted by the sort of parenthesis used between commas in pink.
I'll be pedantic here, but I think the semi-colon, or even a full stop, should be placed right before the fragment in purple.

The paragraph could be clearer, although less poetic, like this:
Cada vez que sonaba un estallido, Platero se estremecía. En el súbito iluminarse* del espacio y en la claridad vacilante, que agrandaba y encogía su sombra (Platero's shadow) sobre el cabezo, yo veía sus grandes ojos negros que me miraban asustados.
*"Se" is used here in "iluminarse" as some kind of impersonal that we use for involuntary events, because the sky glows without any relevant subject actually having control on the light.
Like when you drop something by accident and you say "se me cayó". It's not an intentional thing, and you had no actual control of what happened. You're not guilty; the sky is not responsible for the shimmering effects of the explosions or the noise.

[Quaeso]

Thank you. I'd like to clarify a few things if possible.

"se me cayó"

I'm guessing that se is reflexive here instead of passive, and that me is is an indirect object "it caused itself to fall from me".

but I think the semi-colon, or even a full stop, should be placed right before the fragment in purple.

I'm guessing that therefore you would not have the adjectives azul, rojo, morado modifying estallido, but something else elliptical like [el cielo].

*"Se" is used here in "iluminarse" as some kind of impersonal that we use for involuntary events, because the sky glows without any relevant subject actually having control on the light.

And here I'm guessing that iluminarse is thought to be in the passive sense instead of reflexive, but also I'm not really sure what you mean about the impersonal being used for involuntary events. Maybe like llueve, "it rains" but that is in the active.

[AngelicaDeAlquezar]

I won't make a complicated grammar explanation, because I'm not qualified for it. However, there is a lot more to "se" than just "passive"/"reflexive" functions. You can use those terms, if it works for you to understand the structures, but "se cayó"/"se iluminó" does not fit in any of those categories, even if in English this structure may be translated by the passive voice.

This structure is called "voz media", and it is used to talk about things that change from one state to another, without a specific agent being responsible for those changes.

- La bolsa se rompió. -> Something made the bag to break apart, but nobody is responsible for it.
- La lavadora se descompuso. -> The washing machine cannot decide to break down on itself, it simply went through this change, from working to not working.
- Se me ocurrió venir a verte. -> Something unknown inspired me to come and see you. It just happened to me.


As for the paragraph with different punctuation, the way the author grouped the words confused me for a moment, but I think I get it now.

Here's what I thought should be at first:
<<Platero, cada vez que sonaba un estallido, se estremecía.(Full stop) Azul, morado, rojo en el súbito iluminarse del espacio, (Comma) y en la claridad vacilante que agrandaba y encogía su sombra sobre el cabezo, yo veía sus grandes ojos negros que me miraban asustados. >>
In this proposal, the colors are grouped with the amount of light which lets the person see the donkey is scared.

As the paragraph actually is:
<<Platero, cada vez que sonaba un estallido, se estremecía, azul, morado, rojo en el súbito iluminarse del espacio; (The colors are associated with the explosions, and in the middle of such events, the donkey was startled) y en la claridad vacilante, que agrandaba y encogía su sombra sobre el cabezo, yo veía sus grandes ojos negros que me miraban asustados. (And after the semi-colon, it's how the owner actually can look at the eyes of the donkey, because of the light associated with the explosions.)>>
The paragraph here is divided in two different feelings, the physical reaction of the donkey to each explosion and the actual expression of fear in his eyes. The author's original, obviously, makes the description of the scene much more powerful. I was wrong.

[Quaeso]

Thank you, I like your post very much. I'm interested in grammar, so pardon me if I ever go into too much detail on this website.

This structure is called "voz media"

This is interesting to me because both Latin and Greek have what's called in the grammars a "middle voice" which from my limited experience is fundamentally reflexive (with some minor exceptions). So I'll have to think (and read) about this some more in regard to Spanish.

La bolsa se rompió.
La lavadora se descompuso.
Se me ocurrió venir a verte.

Maybe you would like to disagree, but to me these are all se pasivo. Passive verbs but without an expressed agent.

The author's original, obviously, makes the description of the scene much more powerful. I was wrong.

I agree, and the description is quite vivid, no? So far I really enjoy reading Jimenez, have you read Platero y Yo before, or any other writings of Jimenez? Also it's okay to be wrong, I am wrong very often. But I take my errors like a dose of medicine which help me to learn humility so that I can be a better person, and to serve others and most importantly God to a greater degree than before I took the dose.

[AngelicaDeAlquezar]

Asking about grammar is always alright. We're here to help.

I never learnt Latin or Greek, and for the formal principles of grammar I often have to use the RAE's manual. Since they put "se" in different categories, I prefer to keep them that way, and I try to explain as clearly as I can. In any case, I have nothing against understanding them like passive/reflexive if that's what works for you.
Languages are for communication and that's what matters. Follow-.up questions are usually necessary for difficult topics, and advanced grammar always is. Just ask as many questions as you need.

Originally Posted by Quaeso

I agree, and the description is quite vivid, no? So far I really enjoy reading Jimenez, have you read Platero y Yo before, or any other writings of Jimenez? Also it's okay to be wrong, I am wrong very often. But I take my errors like a dose of medicine which help me to learn humility so that I can be a better person, and to serve others and most importantly God to a greater degree than before I took the dose.

I have a huge cultural gap when it comes to some classics. Platero and Juan Ramón Jiménez? are a part of such gap.
As for errors, well, I'd be far more boring if I were perfect.
Seriously, I agree with you. Admitting to be wrong is good for becoming better.