Moving the color docs to the style guide

[jonrohan]

This is an update to some of the color docs to use style guide jekyll variables added in https://github.com/github/styleguide/pull/232

[mts]

I was looking into this PR out of curiosity.
I noticed the HTML under the headers Gray, Blue, Yellow, Red, Green, Purple
in colors.md are missing the html highlighting.
Is it worth adjusting that in this PR so that the markdown shows up properly ?

[jonrohan]

I noticed the HTML under the headers Gray, Blue, Yellow, Red, Green, Purple
in colors.md are missing the html highlighting.
Is it worth adjusting that in this PR so that the markdown shows up properly ?

@mts we write the html there because we need something extra than we can achieve with plain markdown. For showing these color cards, showing the actual markup is less important. https://styleguide.github.com/primer/support/color-system/#color-variables

[mts]

I understand. Thank you.

[shawnbot]

👌