To achieve this, we have to use the colour palette to set up the original colour plus a lighter and darker version of it meaning three properties for each colour.
I was wondering whether it would be possible to, in the template system, add (or subtract) a 'brightness' adjustment?
I would suggest adding a decimal value because people understand that better.
So, for example, @primaryMedium+20 would add a value of 20 decimal to each decimal of red, green and blue elements. I.e. default Xenforo @primaryMedium+20 would become
#176093 is (23,96,147)
23 + 20 = 43
96 + 20 = 116
147 + 20 = 167
(43,116,167) = #2B74A7 which is a nice, lighter version.
Of course subtracting a value would make the colour darker.
I think this would be a nice and simple way to put what is effectively a brightness adjustment into the template system.