Board Thread:Support Requests - Getting Technical/@comment-31402001-20140220191421/@comment-4018164-20140220212707

Specifying a rgba value would work. For example: Results:

The first 3 values between the brackets are the Red, Green and Blue values, where the last one is the opacity (1 is 100% opacity and 0 is for 100% transparency). On this example it's red with 40% opacity.

You can also use a hsla value instead, which works the same but with the HSL model.

If you're not sure what the RGB or HSL values of a certain color is, you can use an online converter like this one.