Illustrator CS about exceeding gamut warning issues

When using CMYK color mode, a yellow symbol appears on the right with some high purity colors. Domain Warning (click to correct)

He's going to pick the yellow symbol. The green of the purity is not high but if the symbol is not painted with the brush tool, he will automatically adjust the color can not draw a high purity of color. Yesterday with CMYK color mode does not appear this problem, now RPG color mode is not a problem with CMYK mode out of the problem.

The problem is this:

CMYK is used to print a color mode, you do in CMYK mode, when the color can not be printed by printing ink, it will appear beyond gamut warning.