How do I change the color format of a video in ffmpeg?

I have upscaled an old DVD to 4K through various samples and appended the files producing the following x265 file within an .mkv container:

My TV can’t directly play the video, though, usually able to play x265 encoded videos without any problems whatsoever. I believe the problems lies in the color formatting being yuv420p(tv, gbr/reserved/reserved).

Comparing the values with another 4K video also x265 encoded yields the only difference being the color format here being yuv420p(tv).

How do I convert one color format to another in ffmpeg and do I have to re-encode the video (same encoding format, different color format)?