I would strongly suggest to keep the color saturation of the original game for buildings. One thing that always made AOE appeal to me aesthetically were the buildings and units that were clearly visible regardless of background. I consider it a visual staple of the first game and contributes to the identity of the product.
The FPS genre went through its share of gritty realism-induced visuals to the detriment of the color pallete and visual flavour of the games’ landscapes. I hope we can avoid the tragedy of dry, erased-looking colors that make the aesthetics of the buildings and units bland and uninteresting.
Thank you for considering and let me know if you want me to expand on the topic further. If description isn’t enough I’ll cook-up examples by tuning the revealed content to be in line with the original game.
Further opinions along this line can also be found here https://www.reddit.com/r/aoe2/comments/6h88bk/age_of_empires_original_vs_definitive_edition/diwcqtd/
Have a good day!