the highlighter tool (the first one, the yellow one) colors under lines that are darker than the color it has. So if you have a white color on it, it'll draw under a black line. But if its darker than the outline it'll go over it. The second one recolors things the last one is the color dropper

The purple one is an eyedropper. When you wnt to get that same color back but you lost the color in the pallete, just click on the color you want and it is here

The yellow is a highliter. You use it to color. It wont go out of the lines in a drawing UNLESS the color you are using is darker than the outline

THe brown gray one recolors things. It will not make a new line though