Or just type it in manually in BGR24.
draw_set_color($2063A0)
which is just a faster version of
draw_set_color(make_color_rgb($A0,$63,$20))
As for finding the main color in a sprite, I'd draw the sprite to a surface, save the surface to a buffer, then loop through the buffer adding each non-zero color it finds to 2D array or ds_priority, increasing the color count if it's already been found at least once, then destroy the buffer, free the surface, and get color with the highest count from the array/queue.