...BMP format doesn't support CMYK...
...CMYK is an output color space used for commercial printing. Some providers prefer an RGB workflow, whilst others frown upon being supplied with RGB images. With RGB images the provider needs to have all images be converted in RIP via color management settings suitable for there presses and proofers...
...the conversion from RGB to CMYK is dependant on the source profiles and destination profiles. You could have the same image with different source profiles attached, converted to different CMYK destination profiles and the color numbers will be different, resulting in a different image...
...RGB has a wider color range than CMYK so basically those really bright hues in RGB get mapped to a CMYK equivalent....
...desktop printers are OK with RGB as the printer drivers are design to receive it and then turn it into CMYK ink on the paper. A printing press however requires four channels (or more for spot colours)...
...RGB is additive as all three make white, CMYK is subtractive to make white (the paper in other words, assuming it is white paper you print onto)...
...some designers we deal with use RGB images in there layouts, which is OK as they are acceptant of out color conversion profiles...
...one thing you want to avoid, is switching modes RGB > CMYK > RGB > CMYK for example, as this will do all sorts of things to color values...
...some people like to color adjust in RGB (or even LAB for some images), whilst others, like me are more used to color adjustment in CMYK...
...if your project is destined for web use then RGB is the way to go, if for print discuss this with you print provider to advise on color conversion settings to use...
...edit > color settings is where it all happens with regards to working spaces and conversion spaces...
...edit > convert to profile is where you go from one space (your working space) to another (RGB > CMYK for example)...
Andrew
