Explanation

Colorful gradients usually happens when there is a mismatch between the high-poly and low-poly mesh during the baking process. This mismatch can be explained by the following reason :

• The high-poly and low-poly mesh don't overlap properly each other (see image below).
• The high-poly is missing geometry that the low-poly tries to cover.
• The high-poly or low-poly mesh has inverted vertex normals.

When it happens the baking process try to match geometry that doesn't exist, resulting in something empty. The baker fills this empty area with a color extracted from the neighbor pixels in the textures which creates the colorful gradient (unless Diffusion is disabled).

Solution

Given the few possible reasons which lead to non-overlap between the meshes, a few solutions have to be considered :

• Make sure to freeze/reset the mesh transformation (reset x-form, etc) to be sure all the meshes are consistent
• Import both the low and high-poly mesh in your 3D modeling software to verify they overlap properly
• Make sure your naming convention is valid if you are using the Matching By Name feature (you can verify it by baking and then looking into the log file which should print the mesh names).