How to fix this defect?  

RE: How to fix this defect?

Can you marke the problem in your pictures?

Best regards, Clemens

RE: How to fix this defect?

Its the bulge/buldge issue (search for those terms). Its the transitions between different wall structures as it goes to the flat surface.  There are several threads about it.  Basically the fix is to use a different slicer.

RE: How to fix this defect?

@clemens-m

RE: How to fix this defect?

@neophyl

Okay, I will look about it. Thank you very much.

RE: How to fix this defect?

@neophyl

Could you recommend me some slice?

RE: How to fix this defect?

I could be wrong, I only have your image to go by and I can't view it larger or zoom in, however that looks to me like you are using a mildly translucent filament (intended or not) and the areas that are more dense in layers appear darker in color because less light can pass though them compared to the outside parameter. If this is the case then you can either increase the number of parameters to change the amount of light passing though so it will more closely match the more solid top and bottom layers, or you can use a more opaque filament.

If you hold it up to the light this will become more evident if, I'm correct in my assessment.

Bulge is an issue, that generally, when it occurs, leaves and difference in texture that you can feel, and is an actual swelling or protrusion from the surface usually in some small degree, where top layers are formed at some point midway though a model.

Another thing that can happen, in the extrusion process, where the speed of the extruder changes and the filament spends more or less time in the hot end this can have the effect of changing the color and/or the appearance of the texture making the color lighter or darker and relative to texture more dull or shinny.

One more thing to note: if your using cheap filament it could just be poor quality control, many of us have seen this.

Good Luck