It seems that I was wrong when I said that it wasn’t possible with pencil to put a lighter color on top of a darker color. It is possible only you have to prepare to the color you draw first.
Color tree second blend 1th sketch
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You start out with a thick line and blend it with a piece of paper (I suppose a tortillon would also work). After that you can put a lighter color on top.
Even yellow on top of blue doesn’t seem to be a big problem. I never tried it because I was convinced that it would turn green.