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).
Not necessary to pair them with 色. But you can.
Just take 红 as an example.
红 can be used as a noun and also an adjective. Same for 红色.
In English, "red", "green", "yellow" can be used as adjectives or nouns. However, in Chinese, 红 绿 蓝 can only be used as adjectives. For nouns, the 色 suffix in mandatory.
The color of the trousers is green. 这条裤子的颜色是绿色。 (noun) This is a pair of green trousers. 这是一条绿裤子。/这是一条绿色的裤子。 (adjective)