I replaced a fan with a light fixture and the fan used a fan speed controller. It has two red wires going to two black wires. How would I replace the speed controller with a single pole dimmer?
The color (as long as it isn't white, green, or bare) doesn't matter; they're just pig-tails on switch legs. You're good to go here with a direct swap out and arbitrarily connecting the wires to the new device (assuming it doesn't have a neutral contact. If it does it probably matters which contact has incoming power. Read the directions ;).
Make sure those nuts are still snug after you're done.
You did not state what type of light fixture you replaced the fan with. So, you should be aware that the spec on the Lutron Skylark model ctcl-150h-wh states:
Incandescent/Halogen: 10 Watts LED: 1 SSL7A compliant lamp unless stated otherwise in the Lutron compatible bulb list at www.lutron.com/dimcflled"
Most likely this won't be an issue for you, but just in case...