This effect is usually referred to as Gradient Art. Wendy Williams, a long-standing member in the now-closed Elements Village forum, posted this tutorial in her series of tutorials titled "Things Elements Will Do". Here is the tutorial in its verbatim form. Please, make sure to give credit where credit is due when using this tutorial.
Tutorial ... Things Elements will do ... No 1
by Wendy Williams
Well up to now it’s been how to install Elements and how to get it to work ... so maybe it’s time to have a little fun with it .
How about creating something using the gradient tool . . .
Yep, I love it too, and as I recall it was very easy. It's just that my memory is only good for forgetting; you want something forgotten, I'm your man!
You are not alone.... I am right there beside ya!!
Heck, as soon as I sign off, I will forget I did this!!!!!!!!!!!!!
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This is fun with endless permutations depending on what gradient used, what colours used, and indeed on what blend mode used. I used Difference from the Gradient tools menu but also used different blend modes from the Layers Panel on top of that to get the different effects.