The best thing to do is to ask someone to do it for you.
Photoshop is good, but it isn't magic. You need to know how to use it and how the tools work for you to get your desired results. For instance, if I wanted to change the colour of a set of wheels, I would first trace around the wheels with the pen tool (I've never found the wand particularly useful for this) and then transform the path into a selected object. Create a new layer, copy and paste the wheels and then adjust the hue/saturation on that level until you have the desired effect.
For someone who knows what they're doing, it would take 20-30 mins to do this and make it look good. For someone who doesn't know, it will take a lot longer and would probably involve a fair amount of swearing.