This will be a very quick and easy tutorial to follow. I have explained all the steps required to follow the tutorial plus I have provided the resources required for the tutorial
Resources Required
Step 1. Open a new image , the default Photoshop size will do.
Step 2 -Change the background colour to black
Step 3 - Add a new layer
Step 4 - Select a brush and apply the desired effect.
Step 5- Add a new layer use the gradient tool . Select the rainbow and apply the effect horizontally
Step 6 - Change the layer style to - Color
Step 7 - Adjust the brushes layer if you want :)
Voila!
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@mojojojo