Add DevTools-specific issues to visuals/color.

[bwinton]

Which ones DevTools uses in particular, and when, and why.

[violasong]

This shows the current grid highlight color ordering - a special case that doesn't follow the primary UI's gray - blue - magenta color ordering, since for it's an overlay on the page.
https://mozilla.invisionapp.com/share/DGNVPF1TK2Z#/screens