When I still was at uni I had an assignment where I had to colour brackets on a sliding scale.
Something like this:
( // 0x00000000
( // 0x00404040
( // 0x00808080
...
) // 0x00808080
) // 0x00404040
) // 0x00000000
I chose a sliding scale from pure black (RGB = 0, 0, 0) to middle grey (RGB = 128, 128, 128).
Ofcourse, any starting and ending colour works.