I'm making something to teach students Chinese colors... Let's say they have to match a color to a square- Is it okay to just use 红， 蓝， 紫， etc on each block just as long as they know 色 means color... or does it always need to be paired with 色 (ex: 红色， 蓝色， 紫色, etc...)?
I think there are two key reasons to add or not to add 色:
色 adds formality especially for shorter (1~2 characters) color words.
色 can be added or omitted for the prosody of the sentence.
I found a good article on this very topic: 对外汉语教学中汉语颜色词的构成分析. A few points taken from/inspired by the article:
色 is necessary if the word itself is not a color, such as 咖啡色 (the coffee color).
'New age' color words should not be suffixed by 色 (my understanding is they are more like proper nouns), such as 土豪金 (the Tuhao golden)
Reduplicated color words should not be followed by 色, such as 绿油油 (glossy green).