Right, they accept the kanji for green, they can accept 紫色 which is still being marked wrong (1.31.18), and still (2.13.18) sigh still (3.17.18).
And still (7.1.18). I've reported this one half a dozen times already. They'll get around to it eventually.
No, it's not necessary to use "iro" with colors, with a few exceptions like 黄色 kiiro yellow, 灰色 haiiro gray, and 茶色 chairo brown.
I imagine it makes it clear you're talking about the color purple rather something with purple connotations. Kinda like how a "blue person" is sad, not actually blue.