There is no default color space. With the exception of color ramps, the majority of symbols shipped with ESRI software are defined in the RGB color space, so if the current symbol is built from an ESRI symbol, the color space displayed in the color picker is generally RGB. The RGB color spaces exceptions are some of the color ramps. There are four types of color ramps in the ESRI style: Default Ramps, Dichromatic Ramps, Spatial Ramps, and Default Schemes. Default Ramps and Dichromatic Ramps are set in CIE Lab color space, Spatial Ramps in Lab LCH color space, and Default Schemes in HSV color space.
To use only HSV or CMYK color space, or RGB color space for a color ramp, select the desired color space, set the desired color values, and save the new symbol in a personal style. When the color selector is opened to change a color on one of these symbols, the color space defaults to the color space designed when the color was selected.
For new graphics like those drawn from the Graphics toolbar, the Default Symbol Properties can be set to use HSV or CMYK colors. See the link in the Related Information section below for more information on setting the default symbol properties for new graphics.