If you have a float switch on your bilge pump, you'll want a switch that has three positions on it. One will allow the bilge switch to operate the pump automatically. One position will be "off". The last position will be the bypass for the float switch to run the pump no matter what the position of the float switch.
As for color preference, I would say that blue will be easier and "brighter" in sunshine, but red is great for night use. Its easy on the eyes.
As far as the 10 amp or 7.5 amp goes, I would use a 10 amp for the bilge.