The purpose of the camera is to enforce the law. By painting them yellow we're allowing drivers to evade the law: slow down for this bit, keep driving at our usual unlawful too fast speed everywhere else.
Unless you have Orwellian enforcement this is basically not true. People go as fast as they feel comfortable in the conditions. You can't just slap a 50 sign on a highway and expect people to go 50. If you want people to go a particular speed you need to play visual tricks or screw with the traffic flow to make them feel like the desired speed is the speed they should be going. These approaches have their trade-offs but those trade-offs are generally considered to be less bad than the trade-offs from having speed limits that are irrelevant most of the time.