I believe its' still "aoi" for 'Aoi desu' (or in a question form, 'Aoi desu ka').
For colors especially, 'i' must be added since colors describe something. The only exception I know of is "midori" wich already has an 'i', so no extra 'i' is added.
In Japanese, the four colors (赤[aka], 青[ao], 黒[kuro], 白[shiro]) are special. They are nouns, and can be used as i-adjectives, too. Also, 黄色[kiiro] "yellow" and 茶色[chairo] "brown" can act as i-adjectives as well only when with 色[iro.]
So, both "aoi desu" and "ao desu" are correct.
as a noun:
その車は何色ですか？[Sono kuruma wa nani iro desu ka?]
What's the color of the car?
青です。[Ao desu.]
It's blue.
as an i-adjective:
その車は青いです。[Sono kuruma wa aoi desu.]
The car is blue.
Yes. As I wrote in my previous post, they (akai, aoi, kuroi, shiroi, kiiroi, chairoi) are all i-adjectives. So, of course, they conjugate as i-adjectives like "akai," "akakunai," "akakatta," "akakereba"...
みどり doesn't have an い, actually. This is one of the dangers of using rômaji. Remember that consonants must be followed by a vowel in Japanese (with the exception of the very exceptional ん), so there is no Japanese equivalent of "m" or "r," for example.