The first pixel of a flat is not very representative about fastdoom

[AXDOOMER]

Doing an average of all the pixels on the flat texture could be a better idea.

[viti95]

You're right @AXDOOMER (for example OptiDoom uses this idea), but for now it's a good compromise between speed and visual quality. I've tested different values and for now 1850 makes it even better. Thank's @RamonUnch !!

[RamonUnch]

I will try again with 1850. I was also thinking to make the average color but it would be time consuming. the ideal solution would be to precalculate a mapping table flat<->color and load it with fdoom. this table could be even fine-tuned by hand to give the best results.
EDIT: Actually using a small table that contains just a color by flat would be helpful on RAM usage, because for now the whole falt is loaded (4096b) just for 1 color (1b).