A simple way is to add or subtract a little of each of the R,G,B of the color but you can't do that if they are at max (255) or min (0).
Alternatively you can use my class colorpalette
https://www.b4x.com/android/forum/threads/colorpalette-and-circularpalette.32483/
To get the RGB from the selected color (if you have it as an Int) use ParseColor.
Use the method ColorToHSL, add or reduce the L (luminosity, lightness) to make the color brighter or darker, and then go back to RGB using HSLToRGB method.
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