Look at FillCustomPalette for details.

function ImFillCustomPalette(Pal: PPalette32; 
                             Entries: LongInt; 
                             RBits: Byte; 
                             GBits: Byte; 
                             BBits: Byte; 
                             Alpha: Byte): Boolean; cdecl;

ImagingExport (at line 173)