Thanks! It's actually a rgba code, which was implemented not too long ago into profiles. So like, the border to my profile is white, so the code is: rgba(255, 255, 255, 0.6). Then you'd adjust the numbers/decimals until you got the color you're looking for and the opacity.
If it helps, here's a link that explains it way better, with a diagram on how to calculate the codes.
And a Hex to rgba converter for custom colors or colors otherwise not in the above list.