Do you guys really find this kind of tutorial helpful? It is not my intention to criticize, more than that I'm really amazed by your ability of figuring out how to imitate that work just by seeing the steps.
I really will never find out how do you get those results. I mean, is impossible to do that in one layer with only a brush as saphire644 said, isn't it? Or is it because of the software? I only have photoshop so I really don't know what SAI is.
I find it very helpful. I strongly recommend getting a drawing/painting program, rather than completely relying on a photo editing software. PS is good but I feel like a lot of people never learn how to actually draw and paint, and just rely on editing algorithms and shortcuts and techniques to essentially force a blank photograph to look like a picture. Those techniques are important, but knowing how to actually paint is even more important. Remember that if you were actually painting, you only have one "layer" and often only one kind of "brush." And this could easily be done with real paints.
I feel like in the tut, though, the water tool was probably used to touch up some of the brushwork. It could have just been more brushing but I would probably use water. Either way, because I understand the basics of painting and drawing, and am familiar with using digital painting tools rather than photo editing tricks, it's very obvious to me how to get from each step to the next in the tutorial. It's helpful because it's not always obvious how to go from a black circle to the final step. So seeing all the steps is very helpful.
Thanks! I just wanted to make sure I wasn't missing something important that everyone noticed but me but it is difficult (at least for me) to find good tutorials even in youtube. And learning alone seems impossible. -.-
Try experimenting a bit. Most artists don't have a clue how these things work when they start out, but the more you use them and experiment with them, the more you'll be able to find your own way to make smooth colours.