Are 赤, 青, 白 not used without -色 too? 赤[あか/red], 青[あお/blue], 白[しろ/white], 黒[くろ/black] can be used as nouns by themselves with or without -色, and also as i-adjectives like 赤い, 青い, 白い, 黒い etc., while we add -色い when we use 黄[き/yellow] and 茶[ちゃ/brown] as i-adjectives, i.e. 茶色い, 黄色い. 赤, 青, 白, 黒 can also be used with -色, but we don't say 赤色い, 青色い etc. 緑[みどり/green], 紫[むらさき/purple], 紺[こん/navy blue], 肌[はだ/peach/pale orange], 橙[だいだい/orange] etc. cannot be used as i-adjectives; You don't say 緑い, 紫い, 紺い, or 緑色い, 紫色い, 紺色い... You say 紫の/紺の/紫色の/紺色の instead. (Don't ask me why)

@Rib Queen: Ohhh that makes a lot of sense.

all of the colours that REQUIRE "色" have an "object kanji" in front of them, to literally mean "x-coloured" (e.g.: 灰色/鼠色, 黄色, 金色, オレンジ色, 茶色 = grey, yellow, gold, orange, and brown = ash/rat-coloured, sulfur-coloured, gold-coloured, orange-coloured, tea-coloured). none of the "object" kanji are colours on their own, so they require the "colour" particle. 黒, however, IS a colour kanji, which means "black", and ONLY "black". i HAVE seen 黒色 used as a word, but i imagine that it's only as common as we use "black-coloured" as a word instead of just "black" (i.e.: we use it to draw special attention to the strange blackness of the object, in a way that "黒さ" is too mundane for).

黒は別に特別な色ではなかったと思いますけどね。なんの文章ででてきたんでしょうか？例えば、赤や青も色無しで使いますよ。 例：私は赤のシャツを着ている。 例：信号が青になったので進んだ。 黄色だけは色をつけて使いますけどね。 黄だけだと読みが一文字だけなので、他の意味に取られるからかな。 例えば、「黄になる」は文字だけ見れば黄色とわかりますが、音だけ聞くと「きになる」なので「気になる」と普通はとられるでしょう。「黄色になる」だとちゃんと色の黄だとわかりますが。