"Is this room green?"

Translation: क्या यह कमरा हरा है?

[Gemsalike22]

Why can't इस be used here?

[vinay92]

इस is the oblique case form of यह to be used when it is the object of a postposition. यह is in the direct case in the above sentence.

The sentence क्या इस कमरे का रंग हरा है? (which means the same thing) uses इस instead because the noun phrase 'इस कमरे' is in the oblique case since it is the object of the postposition 'का'.

[another-dave]

& the second one, are you literally saying — "Is this room's colour green?", right?

[vinay92]

Yes. Correct.

[Rithuparan]

When is कमरे used in place of कमरा?

[vinay92]

कमरे is the oblique-case form of कमरा. It is to be used when कमरा is the object of a postposition. For example, इस कमरे का रंग क्या है? (What is the color of this room?). In this sentence, कमरे used because it is the object of the postposition का.

कमरे is also the plural form of कमरा. For example, क्या ये कमरे हरे हैं? (Are these rooms green?).
But note that if it is plural and in the oblique case, you should use the form कमरों. For example, इन कमरों का रंग क्या है? (What is the color of these rooms?).