You only need three colors, and if the hot tub doesn't need 120vac, you can use two black for the hots and a green for the ground. So, you may only need two hots and a ground. If the tub needs 120vac as well as the 240vac, then you'll need a white for the neutral.
Don't know if the inspector will buy off on them all being the same color (I doubt it)...too many chances for error or confusion down the road. They sell pvc tubing rated for electrical wiring (stuff I've seen is grey, but they probably make it in others). You'd have to research what size you need for the 6g wires. It has a bell on one end to mate up with the straight end of the next piece. Trying to get 6g wires through conduit can be a pain, especially if it has elbows.