An out of gamut warning simply means the RGB color you have selected is outside the gamut of the document's CMYK working space. If you don't intend to print, this is not a problem.
Also, there's no document color mode with ID. There's a transparency blend space but that just handles the blending of transparent objects-- the document can always have a mix of color spaces. Make sure your blend space is RGB and tun off Overprint Preview and Proof Colors. Overprint On previews pages in the document's CMYK space and the default Proof Setup/Proof Colors is also document CMYK.