Originally Posted by: dreampage 
It's because the lighting model is adjusted for HDR displays. If you don't have an HDR screen you can't see the full spectrum of the picture. Not enough brightness in the sunshine and not enough details in the dark. I agree that on regular screens the game can look dark and washed out, especially in cloudy weather. The track surface and the environment can be surprisingly dark even if it's midday but cloudy. You need an HDR display to fully appreciate the graphics.
This is not accurate. The game
supports HDR, it was not specifically developed for HDR. And the lighting module supports both, independently. I have played the game on 3 different displays (60" HDR 4K, true 120Hz refresh, 60" non-HDR 1080 60Hz, 13" non-HDR 1080 Laptop). And on both Windows and XBOX and it looks very similar across the board. In fact on the HDR set it looked so good in non-HDR for the first few days I forgot to switch over. But once I did it was pretty amazing.
Make sure you have your display calibrated correctly. This does not mean moving a few sliders around until you like what you see. This means getting a calibration disc and running a calibration tool. And adjusting your brightness and gamma (if available) for the proper light level of your gaming environment. Most video games are significantly overly bright and colorful. So if you have one setting for your TV that you're using for all of your games and movies, etc. Forza will definitely look dull and lacking brightness. Turn 10 went for a very photo-realistic visual, not your standard Mass Effect cartoon brightness and color look.
My primary gaming set (the 60" HDR 4K) has been specifically calibrated for gaming. But even with that some games don't look quite right, and definitely have too much brightness. It would have been smart for Turn 10 to include a brightness and gamma setting for non-HDR. They do have these settings for HDR enabled displays and they do make a difference.