Found a solution thanks to Jim Kearns (FoxToolbox.com) and Shawn Dorian/Dan Covill on Ed Leafe's Profox list.

*-- The function "RGBComp" is contained in FoxTools.fll.  laBackColor is the
*-- user chosen background color.

llReturnValue = RGBComp(laBackColor, @lnRed, @lnGreen, @lnBlue)
      
*-- We're trying to obtain the ideal forecolor for the user specified
*-- backcolor.  Thanks to Shawn Dorion and Dan Covill on the ProFox list
*-- for the basis of this solution.
      
lnForeColor = RGB(255,255,255) && Default to White
      
IF (lnRed + lnGreen + lnBlue) > (128 + 128 + 128)
      
  lnForeColor = RGB(0,0,0)     && Black will be more readable here.
          
ENDIF

A tip for those reading this thread - if you need to find the background color on top of a gradient background where the gradient fade is gradual, you can use ThisForm.Point() method to determine the pixel color underneath or near a specific part of your transparent label.

Malcolm