The best solution here is to get a PSU tester like this FrozenCPU Ultimate PSU Tester
. This does several things. It puts a small "dummy load" on the power supply. Running any power supply without a decent load puts a strain on the supply, and can cause it to be unstable. The tester will also give you a pretty good idea if your PSU is working right or not. And it will allow the PSU to power on to test your fans.
Do NOT go by the colors of the wires! Although there is a recommended color coding, it is NOT a standard requirement and not all PSU makers follow it. You can only go by the pin number of the connectors - which is standardized by the ATX Form Factor Standard.
A power supply only delivers what is demanded of it. So in a computer for example, if the motherboard, CPU, RAM, graphics card and drives require 280 watts to run, the PSU will deliver 280W regardless if the PSU is a 300W supply or a 650W supply. And the PSU will draw from wall only what it needs, which will be 280W, plus overhead due to inefficiency, typically another 15 - 30%. The typical generic PSU has an efficiency rating of 70% so it will draw from the wall 364W (280 + 84) with the 84 (30%) wasted in the form of heat.