The best colors for good contrast are either green or yellow because they make the blue lcds look closer to black.  Probably going to grab a yellow one from HHL soon because I like how crisp it looks, and it reminds me of the stock look.

Come to think of it, I have the same polarity issue with a white backlight in a dmg. I just got so used to leaving it in a sweet spot. The contrast on that one isn't real great either, but better than the blue.

It is blue! So this is normal? I tried it first with an orange backlight in a dmg and it didn't have the polarization issue, but it wasn't half as bright or sharp as I expected it to be. Again, nothing like what I've seen from others.

So I put the orange into a pocket instead, and it looks better but not great. I took the blue out of the pocket and put it into the dmg.  The blue looked really good in the pocket, no polarization issue, but was giving me something like 25 minutes battery life.

So a blue backlight really should just have a sweet spot of contrast? And a strange polarity trick?

I had the exact same problem with a blue backlight way back in the day.  Even made a big long post about it, but I think that was on 8bc.  The lcd crystals are blue, so I think that had something to do with it.  I've never seen anyone use a blue light in person, so I was never able to confirm it on someone else's DMG.

What colour backlight did you get?
If it's blue, the polarisation can change when sliding the contrast wheel...

It seems like I read about this same problem on a thread here a long time ago, but I can't seem to find it with a search. I backlit and biverted with HHL supplies, followed the YouTube instructions they link to from This Does Not Compute. As I set it, the polarization should have it normal, not inverted.

Starting with the contrast pot rolled all the way toward the top of the dmg, the faintest shadow of an image is visible. As I roll the pot down, an inverted image appears though not a really nice looking image. As I continue to roll it down, the image almost immediately stops looking inverted and gets to looking almost acceptable but nothing near as good as all of the other modded gameboy a I've ever seen. It basically disappears rolled down anywhere past that.

I feel so sure I've heard of others encountering this same issue. Does this sound like a bivert module issue? Cap replacement necessary? Damaged led ribbon? It' so close to great, I really am puzzled by this. I've got a video of the behavior, I'll try to figure out how to post it.