Learn Croatian: Kada kameleon bude promijenio boju, vidjet ćeš njegove ljuske. - When the chameleon changes color, you will see its scales.

This is the Future II tense, also known as the exact future. In Croatian, you cannot use the regular future tense (Future I) directly after kada (when) or ako (if) to express a condition for another future event. Instead, you use Future II to show that this action must be completed before the main action (vidjet ćeš) can happen. Note that using the perfective present (kada promijeni) is also very common and correct here.

Could I say bude mijenjao instead of bude promijenio?

You could, but it changes the meaning due to verb aspect. Promijenio is perfective, meaning the color change is entirely finished before you see the scales. If you used the imperfective mijenjao, it would mean you will see the scales while the chameleon is in the middle of the process of changing color.

Why is the infinitive missing its final 'i' in vidjet ćeš?

When forming the future tense, if the auxiliary verb (ćeš) comes directly after an infinitive that ends in -ti, you must drop the final -i in both spelling and pronunciation. If you change the word order so the infinitive comes second, the -i returns: ti ćeš vidjeti.

Why does the sentence use njegove (his) instead of a specific word for 'its'?