I believe that you can get higher than 256 colors. The limitation is that LabVIEW deals with images by converting them into pixmap data. It can only deal with certain picture depths which describes how many elements there are in the color table. It can be 1 bit (one color), 4 (16 color), 8 (256 color) or 24 bits which gives you 16777216 possible colors. If you use a depth of 24 bits, you should be able to get all the colors you need.