If you mention the color itself, it is better to say "color + 色" like "紫色" "緑色" "黄色" because some words for color have different meaning when it is used without "色".
緑 means trees, nature. 紫 implies soy souse at sushi restaurant.
赤 can mean correction on composition as "作文に赤を入れる。"
白/黒 implies that someone is innocent of the crime or not as "彼は白だ / 彼は黒だ。"
Only 4 colors can be attached by "〜い".
And the following 2 colors can be attached by "〜色い"