You probably need to do a color space conversion to BT.709. Most UHD content is encoded as either BT.709 or BT.2020 and if it was BT.709 colors would look correct.
The reason colors look pale is because BT.2020 is a very wide color space, 50% red in BT.2020 is much redder than 50% red in BT.709. Without doing a conversion all the BT.2020 color values are simply displayed as if they were BT.709 color values.
Another option is to tag your encodes as BT.2020, for x264 and assuming an SDR source this might be:
--colorprim bt2020 --transfer bt2020-10 --colormatrix bt2020nc
I have no idea how to do a HDR -> SDR conversion.
