About microscope image color problem

I built the Openflexure high resolution type microscope using raspberry pi camera v2, had a problem about taking image.
The attached image looks strange that the color around the center on it is different from the around corner and edge’s.

It maybe caused due to attached 3D printed ~50 mm lens tube but I cannot improve it by making large hole lens tube compared to default stl.
Could you teach the method to improve the problem?