FloodFill doesn't work in 16 bit mode

Hi

My problem is I use an algorythym to make up my custom color pallet. One of the colors has a value of '220' for a paint type program. This color can't be "floodfilled" if my machine settings are on 16 bit color. Instead I just get a dot. It only works on 32 bit. Is there a function, formula, or anyway to convert this number to the closest color that IS in the 16 bit color pallet?

This number is a product of an algorythm using RGB values. How can I insure an RGB value is supported by 16 bit color setting?

Incidently I cut and pasted the color to MS Paint and it filled it without a problem. Please help I don't want to have to rewrite the algorythm using trial and error to find the pattern.

Thanks
Joey O:(