How can I change the colour of a building or car that is in a photo so that I can decide what colour i prefer?
There are a couple of things you can do. Check out the PS Touch tutorial, "Replace colors," as one example.
You can optionally use the other selection tools in lieu of the Magic Wand selection tool.
Also, you can use the Paint Tool's effects options in conjunction with any of the selection tools. Many common effects and adjustments (like "Color Balance" in the PS Touch tutorial) are available by tapping the Mode option and choosing, "Effect," rather than, "Paint."