This is a known defect due to the physical materials using the standard materials from three.js. See end of LDRColorMaterials.js

if(!texmap) {
color.m = m;
}

The problem is that the color of a material blends together with the texture when using the standard materials. The "quick draw" materials I have made for instructions are completely controlled by builinstructions.js, which is why the issue does not appear there. Should we raise this as an issue with the three.js project?

It's up to you to report or not since you're the technical expert and can explain what is happening better than I can.

See also: https://stackoverflow.com/questions/68834918/how-can-i-remove-influence-of-setting-color-in-meshs-material-in-three-js

@Mugen87 is one of the core authors of three.js, and the color mixing is unfortunately intended without the option to turn it off. Adding a check for "inside a texture" as our custom LDraw shaders have, would result in a performance degradation in rendering, so I doubt we will see success in getting three.js core changed. However. We could branch our own version with this feature. The downside is then that the branch has to be maintained by us.

"Rolling our own" Three.js is prolly a bad idea for a number of reasons. I'm willing to live with this quirk as long as you are since LDraw.org doesn't really use physical color rendering in any meaningful way.

I'm still not happy with that. Let me see if I can "hack" the shader generator in any way.