An X display is mapped as a grid of pixels each having a value between 0 and 2N-1, where N is the number of bit-planes available (typically 8, 16 or 24). Hence in a given display only 2N different colours can be displayed simultaneously (in fact it's a bit more complicated than this because different windows can have different visuals within the same display).
In a PseudoColor display (usually 8 bits) there is a lookup table with 2N entries, so that 2N arbitrary different colours can be displayed, each labelled with one of the numbers 1-2N. This is fine until the lookup table is filled up; for instance Netscape might use all the entries to display a nice colourful JPEG image, leaving none for GAIA to use for a starfield which requires many grey levels.
In a TrueColor display a (bitmasked) part of each pixel value is used to determine how much red, green and blue comprises the colour. Thus in a 24-bit TrueColor display a pixel value of 0x0080FF (hexadecimal) would usually mean a colour with no red, half intensity of green and full intensity of blue. If you have enough bits (and 16 or 24 is enough for most purposes) this performs well at displaying any image, and there's no colour map to worry about filling up.
Using VNC to display Starlink graphics applications