Please read this first: 赤ずきん vs. 赤い頭巾 (grammar)
So when someone says 黒タイツ, the speaker is using it as an established concept/genre in the field of clothing. That is to say, just by hearing it, people can imagine what else the person is wearing. If you want to simply say black-colored tights unrelated to such an image, you can always say 黒いタイツ. The same for 白タイツ, 網タイツ, etc. Some combinations such as 黒服 and 赤帽 have derivative and unpredictable meanings.
I personally have never heard the phrase 紫タイツ, but if purple-colored tights should happen to catch on in the future, people may start using 紫タイツ as an established term. On the other hand, many otaku already regard ピンク髪 as an established "moe trait" of anime characters.