What picture settings are you using on the ATV? Do you have the issue with any other apps?
I would recommend resetting the ATV picture settings and then setting the ATV to output 4k SDR. Then, if you enjoy HDR and DV, set both of the "match content" settings in the ATV to "on". HDR/DV will then be output when appropriate (when the source content is HDR/DV), but the ATV will output SDR when the source content is SDR (i.e. YTTV, etc.). Personally, I don't like HDR/DV on my Sony OLED (too dark for my liking when comparing the same content in SDR), so I have the "match content" settings set to "off" so that the ATV only outputs SDR.
I installed the YTTV app and signed up for a free trial and compared the picture quality side by side with Hulu Live TV and AT&T TV Now and none of them were washed out when streaming to my 2018 Sony A8F OLED. They all looked similar, although YTTV's picture was just a tad softer/more compressed/less sharp vs. the others when watching identical content (I like CNN newsroom content purely for comparing PQ). ATT TV Now had the best PQ but all 3 were pretty good overall and better than my Cox cable PQ.