On the MGF, It is white with a black strip (this is 'standard' colouring in the automotive world). I have a shift light and picked up the RPM from this cable going to the driver's fuse box. You can also pick it up at the MEMs as well.
I have a shift ECU that I bought back in circa 2000 which was a kit semi supported by MG-Rover. The LED alarm light acts as the change-light and I can set it at 3000/4000/5000 RPM - very neat installation.