One way of pulling two colours together is by making very small marks of colour starting heavily on the left and gradually dispersing them more widely as you move to the right. Then do the same thing with another colour from the right towards the left. If one colour is a lot stronger or darker than the other you may have to fade it out more quickly. In my examples, I've done from yellow to red and from red to green. Note that the yellow-green was helped a bit towards its stronger end by another deeper green.
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