This brings up another point. If you have studied the resources on this site about CMYK, RGB and how they relate to each other (provided in the profile for Lesson 3 of the Adobe InDesign Basics class), you will understand that there are colours that are in the RGB spectrum which cannot be made by mixing CMYK.
If you are using your Colour palette in RGB mode, InDesign will tell you when you have made a colour which cannot be reproduced exactly once you convert it to CMYK and will show you the closest colour. If you look at the illustration below you will see the Colour palette in RGB mode. At the bottom left of the palette, just above the RGB spectrum, you will see a yellow triangle with an exclamation point and a small swatch. The triangle tells you that the colour you are mixing in RGB cannot be reproduced in CMYK and the swatch just next to it tells how the colour will actually look when printed.
