You can do it like this:

define detect color
go to [mouse pointer v]
set [color v] to (0)
repeat (32)
repeat (32)
repeat (16)
if <touching color (color)?> then
stop [this script v]
end
change [color v] by (16)
end
change [color v] by (1792)
end
change [color v] by (458752)
end

Make sure the sprite this custom block is in is set to a 1 pixel costume. This works by cycling through all possible colors, checking for each one. To draw it, just do something like this:

... :: hat
set pen color to (color)
pen down
pen up
...