The Sketchup OpenGL shaders apply different specular lighting to plain-color versus textured geometry.
In this example, the lower half of the sphere has been textured with a bitmap of the same color as the upper half.
The delineation between color and texture is clear where the specular lighting is more prominent.
The different shading is still present without shadows turned on.
This should be a simple fix for the Sketchup developers.
pls attach the test model to the thread (or the original post using the pencil icon.)
[If on Windows you can just drag and drop allowable files into the post edit box.]
Basic sphere model attached:
Sphere.skp (437.3 KB)
It looks the same in my SketchUp too. I don’t know of a remedy, but I have made an observation that photorealistic renderers, too, behave differently with textured and nontextured objects, and generally you get better results if all the faces in your model have a texture.
Anssi
I believe this has to do with how lighting affects colors vs textures. A texture and color react differently to the settings in the SHadow window. Colored surfaces can be almost completely washed out by increasing the Light value, while textured surfaces keep their color.
I cannot say WHY this happens, but this is WHAT is happening!
That is why I believe it has to do with Sketchup using different OpenGL shaders for the different materials. I have played around with shaders - my profile picture is a Centaur created in sketchup, but rendered using my own software written in C# with OpenTK, to achieve the per-vertex coloring necessary for the blend between skin and hide.
If the texture shader and the color shader do not share the same code for lighting, then this is exactly the effect you would get!
It is still a bug in Sketchup, because materials of the same color should look exactly the same.
It should also be trivial for the Sketchup developers to fix.
Kerkythea renders the textured and non-textured surfaces as the same colour, and this is the only renderer I’ve ever used. I know that Sketchup isn’t meant to be a renderer, but I’d really like my models to look good in Sketchup. 
For the Centaurs I’ve previously posted to 3d Warehouse (mostly taken down now, and I’ve lost my account details anyway), I created a ruby script for the gradient between skin and hide, which looks good in Sketchup, but looks terrible if rendered.
Without per-vertex colouring, I need to texture the models, but then they look washed-out in Sketchup, plus a fully textured model is a lot bigger in file size due to all the UV coordinates. The best of both worlds would be if Sketchup displayed textured surfaces with the same specularity as untextured surfaces.
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