BMP and PCX use 16-bit color. BMP has support for palettized graphics. If your BMP file is palettized, open MSPaint and the image, go to file > save as (no need to make any modifications), then, in the drop down menu, make sure 24-bit bitmap is used, then save it. The file size will get bigger, but this will fix the colors.

TGA is the only 24-bit color format in the engine and it also supports 32-bit color.

In 6.4x, however, BMP and possibly (untested) PCX also allow for 24-bit. I don't know about panels though.