this is the true color representation. but the problem is a 1000*1000 true color image would take up 3 million bytes and also if each pixel in this image had a different color, there would only be 1 million colors so this representaion of 16.7 million diff colors is somewhat not practical.

 Hence we have LOOKUP TABLE . in this the pixel values do not code colors directly instead the have the addresses or indices into a table of color values.

there is a table having 256 entries each of 24 bits color value(8 bits per primary). Pixel values are now 1 byte telling number from 0 to 255 in the table where a color value is stored.

so a 1000*1000 image now requires 1 million+768 bytes( for the color values in lookup table). hence it allows 256 simultaneous colors that are chosen from 16.7 million possible colors.