Boat to land collision

I'm having problems or don't understand the values of readpixel() is supposed to return

From the command help it is supposed to return a RGB value, like 255,255,255 for black, but I get other numbers like -1 for this one.

I would like to compare an RGB value against the value of readpixel() to see if there is a collision with one color. this will allow me to run different operations when colliding with different color pixels.

Thanks!