It is best practice to have Mobile HDR off when developing for Quest - With mobile HDR off it disables post-processing on mobile.
The ‘flat plain colors’ that you are seeing is a result of not having the bloom post process effect.
Like Bernhard mentioned, you’ll see a better representation of what to expect on mobile devices by changing the ‘Preview Rendering Level’
But feel free to enable Mobile HDR and try deploying to Quest, with enough optimization you might be able to get away with the added post-processing overhead.