Hey damonjynx, i know well that shading without a tablet is quite ennerving, but take a look at this awesome tutorial by torstan (if you don't know it already) "Dungeons! Using Layer Effects, Stroking Walls and Going Old School TSR Blue". You can do a lot of the lights and shadows with just stroking paths with different brushes. As Torstan says in his tuorial: "creating dungeon maps without actually drawing anything". XD
Ps: I too prefer to color my maps mostly with overlay or multiply layermodes. I only test printed some of my maps and also found that they often come out quite dark, but i really haven't tested it that much.