Wow. I've had the exact same LCD issues since the first day I got my KeyLab 49 mkII. There's a faint zombie green glow "behind" the blue light.
My theory is that they used cheap 2x16 LCD displays which are available with blue or green background + backlight. However, the internal backlight color tint of the "blue" models are different than the "green" models, simply to make the text readable.
I won't go into detail, but the blue displays have light/bright characters (negative layer), while the green models have black/blueish text. So if you switch the glass piece layer, you end up with a blue tinted display with the green background showing through in the areas that the backlight can't fully reach/illuminate.
The reason I know this, is because I did some experiments with different LCD displays on a Raspberry Pi and Arduino, with the exact same symptoms/result/issues as on the Keylab MKii.
I live in Norway and I ordered my keyboard directly from Arturia in France, so I never bothered to call support or anything. Just seemed like a lot of hassle with returning it and everything. However, it has bugged me since day 1, and it's the only reason why I rarely use that keyboard 😕