We know that the color of surfaces can be reflected onto other surfaces, seemingly dependant (from my observation) on strength of light, closeness of surface to the other surface, and smoothness/roughness of surfaces.

-> Where a smoother surface would reflect more of its' color onto another surface, than a rougher surface, under the same light source/strength of light.
-> And a smoother surface would recieve more color off another surface, than a rougher surface, under the same light source/strength of light.

-> And the stronger the light hitting a surface, the more color would be reflected off it.

Then it seems to me:
-> The closer a surface to another surface, the more color would be reflected onto the other surface. (At least on rougher surfaces)

And that last point is also one of the reasons for my questioning.
So surfaces always have two properties regarding light, determining their look.
-> One property is: Which colors, which wavelengths of light it absorbs the most, and following from that, which colors it will reflect to the viewer, giving it its color.
-> The other is: How specularly reflective the surface is. Depending on surface roughness/smoothess. How much more mirror like the surface reflects light. Being more rough, having more diffusion and surface angles into different directions.

These two properties are, if I don't remember it wrong now, coming from a more microscopic level. Depending on the density of a surface, color is given from light entering a surface, being diffused and coming out again, while some spectrum of light gets lost inside. And if the surface is completely dense, we would have a mirror.

Please do correct me if I am wrong.

So back to my problem again. When we know, light comes in in one angle, and goes out in an angle to the opposite direction, which we know from the behaviour of light looking into a mirror, every specular reflection should be acting this way. Which means, the specular reflection on a surface changes, depending on which angle it is viewed from.

Now, I have observed exactly this in some tests I made with surface color being reflected onto another surface, especially in matte surfaces. Where I could explain the behaviour of the reflected color, with specular reflection.
For example, changing my viewing angle to a matte, colored toy cube, laying on a white piece of paper, and the reflected white from the paper on the cube adding up, and lessening respectively.
BUT in some cases, I was totally lost as to why the light was reflected the way it was.

Here are two cases, I couldn't explain:
We have blue curtains hanging closed in front of our windows. When viewed from the side, there is some space between the wall next to the windows and the curtains, so light can shine in from the sides.
I can see blue light is reflected from the curtains onto the white wall, depending on how much I close the curtain (move it closer to the wall, more and more). That got me wondering, how is this possible? When I explained to myself earlier, that the surface color reflection onto another surface can only be explained by specular reflection, otherwise the other colors would be absorbed. Because no matter how I changed my viewing angle, the blue light radius on the wall always remained the same. And especially, there shouldn't even be any blue reflected light from the angle that I was looking.
Is this explained by the roughness of the surface of the wall? Making the blue reflected light be reflected back into my viewing angle, no matter from where I was looking, because of it's diffused surface angles?
Which would basically mean, specular and diffused reflection are working together here, to give this effect?

The other was this:
I have matte cube again, or any other matte surface, in this case standing vertically in a 90 degree angle to the floor.
Now I slowly move some white paper on the floor closer and closer to the vertical surface. What happens, the closer the paper gets, the more white color is reflected onto the vertical surface.
Again I struggled to explain this to myself. If I had moved the paper away from and closer to a complete mirror, there would be no change in light strength reflected back to me, of course because a mirror reflects 100% incoming light. Now the only difference a non-mirror surface has, and as good as every surface has some degree of specular reflection, higher or lower dependetly, would be the roughness of its surface, also determining afore mentioned specular reflective strength of surface. Nevertheless, it would be reflecting specularly, just weaker, or stronger. So I can't completely understand, why the reflection of the color of another surface seems to get stronger/weaker, depending on how close it is to the other surface?
I mean, what seems to happen is, the rule of strength of light, the amount of light per area unit. As a piece of paper would reflect its' white light diffused, into many directions, the further away I move the paper from the other surface, the less light per area unit would hit the other surface. But then again, wouldn't that mean, the more I move the paper away from a real mirror, the darker it would become? Which is not what's happening. I think I am going crazy. Or is it, because the specular reflection of the paper on the matte surface it not so strong to begin with, so moving away, the paper becomes smaller, giving the illusion of less, but that can't be, because of what I just said before, with a real mirror. That shouldn't affect it. Or that in combination with the roughness of the surface recieving the white of the paper, diffusing its' reflection. It's a little much in my head right now.

To think, that in the beginning I thought reflected color is such a simple thing, because I just accepted a simple explanation I once got from a teacher. But when I started thinking of more diverse situations, that explanation seemed like basically just saying: "Light hits a surface and reflects color to another surface, if You look closely", lol. Without actually explaining how it worked.
There are so many things to consider, influencing it.


Thank You in advance, any help is greatly appreciated.