Do gems scale with their corresponding color?

Do the color of the gems just tell you which attribute they're linked with (so, for example, if you're a witch, using a red gem could be possible/helpful, but you generally want to stick to blue).

Or do they scale with your attribute, so the higher your intelligence attribute, the more damage/effect your blue gems deal?

I ask because I received a red support gem as a Shadow. I generally avoid the red gems since I am the Blue/Green class, but this one is actually going to benefit me.. But would it benefit me more if I had more strength?

Only stat requirements, that's all.
If your witch can get 90 strenght, she can use some red gems (and she most likely will use at least Reduced Mana gem) but if she would have 1.000 strenght somehow, it wouldn't make red gems mor epowerful. Only levels and quality of the gems make them more powerful. Not stats on your char.

I have this on my ranger for example

Thank you guys, that clears it up for me :)

I'd like to point out for completeness however, that strength does in fact give 1% increased physical damage for every 5 strength points you have, so physical based melee skills do infact get stronger the more strength you have and many of those skills are red gems.