I am about to give you simple solution.

At first bitmaps must be external files (excluded from project and distributed with executable). It's better to use temporary files. (Editing files in program dir is not allowed for normal user accounts on NT and higher systems). Thus you need to dynamicaly assign Picture properties. :(((
But you can code this once only in parent class.

At second you may not edit all pixels of picture while you can only change palette color from black to what you need.

This example code replaces "black" palette color in source file with nColor and saves modified picture to target bmp file.

LPARAMETERS cSource, cTarget, nColor

#define StartPosition 55

LOCAL lcBMP, lcColor
lcBmp = FILETOSTR(cSource)

lcColor = ;
    CHR(BITRSHIFT(BITAND(0xFF0000, nColor), 16)) ;
  + CHR(BITRSHIFT(BITAND(0xFF00, nColor), 8)) ;
  + CHR(BITAND(0xFF, nColor))

lcBMP = STUFF(lcBMP, StartPosition, 3, lcColor)

STRTOFILE(lcBMP, cTarget)

Hope this helps