Graphic images that overlay a transmitted video signal are provided in the form of bitmaps that allow any character or font size to be displayed. A color-look-up-table (CLUT) at a receiver is defined that maps “m” inputs to a combination of color and luminance values for display. One or more map tables are provided that allow pixel colors to be encoded using fewer than “m” bits, the map tables providing an “m” bit output for accessing the CLUT. The provider of the graphic images can also specify a required minimum number of CLUT colors for rendering the images; if this minimum is greater than “m”, the image is not rendered. The use of map tables provides compatibility between a variety of encoding techniques and a variety of rendering devices, without requiring different CLUTs to be transmitted for each different color encoding format.