"Chan eil falt ruadh ort."
Translation:You do not have red hair.
dearg is the red of blood or saturated red paint, ruadh is the red of copper, rust, red hair and fox’s fur¹. Those are two distinct colours for which English doesn’t have separate words.
I’d recommend you to just learn that red of hair or animal fur is ruadh, but generally red is dearg.
¹ fox is even called madadh-ruadh, literally red (haired) dog, in Gaelic (or sionnach, which is an alternative word for fox)