When you say compare, this means that you are looking for things that are the same and different between the two items. Sometimes you can also compare more than two items. When you say that you are looking for contrast, this might mean that you are only looking for some differences between the two items.
Once again, you can also contrast more than two items. There are some people who say that it will be hard to check out two or more items and not look at what their similarities are. There are just times when they can be so different from each other. The best way to use compare and contrast is to look for items that will still be connected to each other or will have some similarities so that you can check some unexpected similarities.