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If you want to do it right, it is going to take some time. There is no way around that, unless you want to use some kind of junky instagram type filters with results that will reflect your lack of effort. Here is a step by step guide. Just take your time, and it should look pretty good at the end. I've seen some that are really convincing. You can also use GIMP, but here is a good walkthrough on how to colorize using photoshop. Hope it helps.