Corrected sentence (clear):
When an agama gets hot, or when it is excited or angry, its colours get very bright.
Even simpler (for a 4-year-old):
When an agama is hot or angry, its colours get bright.
Notes (step-by-step, easy):
- Step 1: Say who you mean — "an agama" (this tells us the animal).
- Step 2: Say what happens — "gets hot," "is excited," or "is angry."
- Step 3: Say what changes — "its colours get bright." ("its" means the agama's colours.)
- Spelling note: In British English use "colours." In American English use "colors." Both are correct.
So your original idea was good. The corrected version above is a little clearer and easier to say.