1. It will be almost impossible to reproduce the exact coloring in any given icon if the picture you are using isn’t from the same scene or photoshoot, so give up this fantasy right now. Be inspired by others’ coloring jobs, but do not look to duplicate precisely because you will just drive yourself crazy. Try to distill what exactly it is you like about the coloring – is it the soft glow, the high saturation, the blue tint, the retro feel, or whatever? Then look for tutorials/advice on how to recreate this general feeling within your own graphic; you will then end up with an icon of your own making, of which you can be much prouder than just a ripoff of someone else’s.
2. Selective coloring/curves/levels/certain textures/brushes/anything else that seems ubiquitous and complicated is not the answer to all your graphic problems. It is entirely possible to make satisfying icons without the use of these things. So, if your program doesn’t have a particular function or you don’t understand how to use something, do without it for now – you’ll be just fine. A particular texture or a selective coloring layer isn’t going to take an icon from F to A+ in one fell swoop, especially if you don’t have good grasp on how to properly use it, so let it go and just concentrate on what you can do in the moment.
3. If you’ve spent three hours (or really anything over 15 minutes) on a graphic and you totally despise it, for the love of jeebus, stop working on it this instant. Either start over at the last point you liked it, with the original picture, or an entirely different one. Stop and think about what it is that you don’t like and what you want it to be like instead, so that you can have an aim while you are playing around with it. Say to yourself “I don’t like how grungy it’s become; I want clean lines instead” or “I don’t like how yellow his skin looks; I want to keep it natural tones” or something, so that you can evaluate each change you make as positive (towards your goal icon) or negative. Then, scrap the unconstructive adjustments and try to stop yourself before you waste time playing around with them. For instance, don’t open that grungy texture if you want a clean, crisp icon, even if grungy textures (or whatever it may be) are the hottest thing on the scene right now.
Having a particular aesthetic that you are working towards isn’t entirely necessary, but generally helps in finding that point of declaring an icon finished, which is good for people without a lot of time to kill. However, for someone just exploring the graphics-making world, opening a picture and playing around without a goal in mind is a perfectly acceptable and wonderful way to learn your way around a graphics program, as well as discover what you like or don't.
4. Go forth and have fun. If you are in this for fame or compliments or anything that doesn’t also include pleasure in the process, get out of the business right now because it’s not worth the trouble (because icons are serious business!). If you just want an icon that looks a certain way, but don’t really have any interest in creating it yourself other than being able to say you did, go ask any one of the many, many talented icon-makers out there who take requests. Please.