You might be able to make a potential divider with the rev counter wire as the input (not sure how that would effect the actual reading, maybe make a parallel wire to it in case?). If you use a variable resistor you can fine tune the circuit to get to the right revs.
Edit: cant's seem to upload the image or a link to the image, Google image search "potential divider variable" instead
The output could then trigger the switch between blue and red lights, with a transistor or relay