Strange effect on the solid color faces I'm trying to achive something really simple but I've got the annoying effect. I'm using Cycles for rendering. On some meshes I applied the solid color wihtout any special effects (just the Diffuse BSDF with the chosen RGB color). I have one point light and a surface shader set in World Properties. The area on which the strange effect appears doesn't have multiple faces, in which case some problems with normals might appear.
I even didn't know how to name this visible effect, so it was difficult to find the solution somewhere. It's something like a shades along the faces. If there is already an answer on this forum, sorry I didn't find it - please direct me to this answer.

Thank you in advance for any help. Probably it is something very simple. But I don't have much experience in Blender, especially in materials and rendering.

Mariusz
This looks like denoising artifacts. Denoising is active by default in newer versions of Blender, and a quite powerful tool. It smoothes the noise that you get when rendering, and can save some render times, but results in some "washed out colours" if rendering with too few samples. Try increasing your render samples, and try deactivating Denoising for testing purposes to see the difference.