cclrUsed (ULONG)
The number of color indexes from the color table that are used by the bit map. If this is zero (the default), all the indexes are used. If it is non-zero, only the first cclrUsed entries in the table are accessed by the system, and further entries can be omitted.
For the standard formats with a cBitCount of 1, 4, or 8 (and cPlanes equal to 1), any indexes beyond cclrUsed are invalid. For example, a bit map with 64 colors can use the 8-bitcount format without having to supply the other 192 entries in the color table. For the 24-bitcount standard format, cclrUsed is the number of colors used by the bit map.