Here is some additional infos:
1 – I noticed that native firmware starts in this dimmed state, then when it switchs to the Menu it appears with no dimming issue. Is this normal with your Bricks Native Firmware?
2 – With Mono Brick firmware, It starts dimmed then the Mono Brick logo appears in a dimmed display too, then the Mono brick menu appears and stay in a dimmed display.
Please note that in between I tried other SD cards & I tried All images, and I have same symptoms.
Regarding your question, what happen when you write to your LCD etc…,
with Mindstorms programing blocks writing is OK.
With Monobrick firmware example I guess Color Sensor writes some text, but I wasn’t capable to see what is dispalyed it’s why I tried Motor example to see Motos running it was better 🙂
But regarding point 1, It seems that’s a brick issue.