Black Construction Paper vs Mirror Card

Anytime you change the background you should probably re-calibrate the scanner to ensure accurate colors. You’ll see the same issues switching from the standard while scanner lid to the black construction paper.

I should receive the Mohawk Chromolux today and will see how it does.

Great pics! I read with interest the earlier discussion, and then into this thread.

I don't use black construction paper. I tend to use smooth black cardstock (letter size typically suits my needs). On the full-sized mint sheets/panes, sometimes I will use the black poly-pro backing in my mint sheet albums. However, the poly-pro is really soft, so you can't really tape it tight to the lid. Since my scanner doubles as my copier/printer, I just leave the smooth black cardstock loose and simply place it on my stamps when I want to scan them. The advantage is that I can actually use the black cardstock to "push/line-up" a whole row of stamps (this is in the days before PostmasterGS StampFix). I'm so unprofessional!

Regarding the color shade differences when using different background, that is actually a fundamental artifact of all optical sensors, including the human eye. Optical sensors operate logarithmically and sensitivity/response spectrum will overlap. So certain combinations (even if not at saturation levels) can affect optical sensors, and thus noticeable by the unaided eye. The grayscale transition from black to white is an example. It's a hardware limitation, not a problem of the software. To correct it, you actually have to use software (or convince/train your brain that it's actually a deeper orange).

I once gave a short talk about how background colors affect perception, and how the brain compensates, and how to fool the brain into incorrectly compensating. I fooled most of the crowd (90%) into thinking a 2 brown tiles were 1 brown and 1 yellow, simply by putting other colored tiles next to it. I then fooled them into thinking a pink ball was turning green. Actually, all I did was take away the pink ball -- there was never any green ball. 10% of the crowd guessed correctly, but that's because they figured out I was deliberately trying to fool them, so they simply guessed the opposite of what most others were saying.

That's a nice comparison, youpiao! So, which one actually looks like the color of physical stamp? (not that my monitor is color calibrated!)

What's really interesting is that in your example, the mirror background makes the stamp colors lighter. In my example, it makes the stamp colors darker. Just goes to show why color in scans and on monitors is a crapshoot.

Here's a situation where the reverse happened. The scan on the mirror background came out darker than the one one on the matte background. Evidently, it's not so much the background affecting the color as the scanner's varying sensitivity to different wavelengths.
And, again, the darker scan looks closer to the real stamp.

Yes, scanner response is not uniform. However, same stamp, with only thing different being the background. Therefore, the background is the primary factor in causing the optical sensor to "see" differently.

Different color stamp, same background, but reversed results, then it's the optical sensor response that is dominant factor.

If same stamp and same background, and picture still came out differently, then we go into lamp/sensor stability. We typically had to have the lamp/laser & sensor equipment well-warmed up to steady-state conditions before taking measurements.

For scans, I typically turn on and scan. The Plustek (at least mine), does have a warm-up delay at start or after long pauses between scans.