This message is generated if Q++ notices that a GridTemplate you have scanned has more than 20 colors defined in it (this does not include the colors used by any images in the page). The more colors are defined, the slower the page copying stage of diary generation will be.
Note that a grid which contains the Registration color, all the pre-set colors (CMYK, RGB and White), plus 3 other colors user-defined colors, would still only amount to 11 colors. So a limit of 20 colors per grid seems more than reasonable.