I find it very hard to shoot colour and B&W at the same time; ie, switching between them. I do better when I do one or the other and focus on that. I just see differently when I shoot one or the other and when I go shooting for myself usually pick which I will shoot before I start out. The pictures benefit from that.
When shooting film for clients, and both were wanted, I always tried to determine which was the primary requirement for them if a single shoot had to include both.
Since almost all digital cameras are set up basically for colour, and most can't even show you a B&W image on the chimping screen when you shoot RAW, I only shoot B&W with the Oly EP-1, which works slightly better in B&W than most digital cameras I've used, or I shoot film. The M9 I use almost exclusively for colour. The M8 gets used for B&W at times, but only in IR.
Henning