Good tip, Vojo!
The ShapeSheet also has HSL functions (hue, saturation, luminosity), which I think are easier to use for creating lighter/darker shades. For instance, you can make a darker shade of red like this:
User.Red = RGB(255,0,0)
User.DkRed = HSL( HUE(User.Red), SAT(User.Red), LUM(User.Red)*.9 )
Note that luminosity values are good from 0 to 240. 0 makes black, 240 makes white. 120 is full saturation. So LUM(RGB(255,0,0)) = 120.
If you add or subtract values to make your shades, you need to ensure that they don't go below 0 or above 240.