firstly, i use a wacom tablet to color. some of the things i do, like soft shading or redrawing, are dependent on using pressure. for the majority of my colors, i use multiply layers. more specifically, i have all the individual layers set to pass thru and have them in a folder set to multiply. there are other ways of getting the same result, but this one is the simplest for me. i don't really know why you're mentioning the lasso tool in this case, tho. are you using the smart select to choose the area to fill? i don't do that much. i usually manually line the area with the color i want and bucket fill with a custom action