I think the essential difference - but please correct me if I am wrong - is that RGB (Red, Green, Blue) is additive colour mixing where each colour you add to the mix adds colour. So, if you add a red light to a blue light you get a yellow colour. If you add red, blue and green you get white. So, it is used for screens, where light is created.
CMYK (Cyan, Magenta, Yellow, Black) is subtractive colour mixing where each colour you add to the mix subtracts colour. If you add cyan and magenta you get blue. If you add cyan, magenta and yellow, you get black. Subtractive colour mixing is used for printer inks or paints and commercial printing and publishing companies sometimes ask for CKYK to be used.
So, what does the CMYK option in Tools > Options > OpenOffice > Colours do? And how do you use CMYK if your printer or publisher asks you for CMYK?
Help says
Does this mean that you can specify a colour using CMYK values, but that when AOO prints, it will output the RGB values which are equivalent to the CMYK values you entered? If so, then any colour stored in a .odt file will presumably be stored as RGB and not as CMYK.Colour table
To modify, select the colour model: Red-Green-Blue (RGB) or Cyan-Magenta-Yellow-Black (CMYK).
OpenOffice uses only the RGB colour model for printing in colour. The CMYK controls are provided only to ease the input of colour values using CMYK notation.
If that is right it suggests the conventional wisdom that AOO only supports RGB is correct.
Also, if you specify a colour as CMYK does that mean that the RGB colour created and stored by AOO can be converted back to CMYK without any colour changes? So, if a printer asks for CMYK, could you use CMYK values to define the colours you use, and tell the printer that all your colours are defined as CMYK colours, but they are expressed as RGB colours? It would work for text and graphics created by AOO, but images, photos, graphics or text copied into AOO may not use "CMYK" defined colours.