It's just something that improves over time with lots of work and practice. When I started it would take me all day to color a pinup. Now I can do the same amount of work in about an hour. Several things that slow you down at first just end up becoming second nature over time. Practice practice practice.
So how have you found it easiest? One of my friends swears by making channels for different sections that need colour which she learnt from some guy that does art professionally, and some change the line art to a multiply layer and colour on the higher or lower layers, like me. Do you do either of thouse, or have you got a different way? I'm curious cause I'm sure I could be alot more efficent than I am now.