That's not specific enough a description. There are 16777216 colours to choose from, and countless ways to pick random colours from that set. You have to understand your needs first, then devise your own strategy as to which ones you want and pick accordingly.
Assume
image_blend for the target, and that you have already called
randomize() as TsukaYuriko instructed. Here are 3 common methods:
Picking from a predefined set:
GML:
image_blend = choose(c_red, c_yellow, c_green);
Generating a random colour (This is equivalent to TsukaYuriko's method using
irandom(c_white)):
GML:
image_blend = make_color_rgb(irandom(255), irandom(255), irandom(255));
Generating a random colour of uniform saturation and value (i.e. picking from a rainbow):
GML:
image_blend = make_color_hsv(irandom(255), 255, 255);