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First use the gradient tool in black and white using the conical (Asymmetrical) shape (This is in The Gimp.) to make a gradient with the mode set to "normal". Change mode to "difference" and then using the same shape, lay another layer straight over the first layer, not using a new layer, just as if you are painting one thing on top of another. You should get these kind of patterns, which you can play with further by cropping along lines or in halves or whatever then laying two identical halves together, etc etc. Then add a transparent layer (this is an actual new layer and you need to pick a mode that allows the top layer to interact with the bottom layer, not just block it out, difference mode or addition or whatever, but you can change the layer mode once you've got a colour laid on the transparent layer, to see how the different modes interact with the layer below.) and on this new layer select a coloured gradient, again using the conical asymmetrical shape and lining it's starting point and gradient "line" up with the lines in the image you see "underneath" your transparent layer. The colours will interact with the layer below. One such experiment produced this.
Enjoy.
Mikel.