Working with VF5US images for use in diagrams & presentations, I thought I would do a side by side comparison between their counterparts in VF5FS.
US character images (from 1080p) was scaled down to match FS characters' size (720p). Ultimate Showdown images were taken from the retro stage, while the Final Showdown images were from the training stage, iirc. That might be a factor on the warm or cool undertone differences. While only 720p, VF5FS graphics has aged well in my opinion.
Some of the more obvious differences:
1. Hair! US characters seem to have went to the barber shop/salon to gt their hair highlighted/stylized before fighting
2. US and FS seem to prioritize differently what to contrast. In FS, there is more emphasis to contrast skin. Especially noticeable at the jaw and neck. In US, the contrast is emphasized on clothings. Look at the "inside" portion of clothes, for example the interior of Akira's gi.