This is covered as a policy check and not an implementation check.
ffmpeg -f lavfi -i testsrc2=s=120x80 -f lavfi -i sine -c:a flac -ar 8000 -vframes 2 -c:v ffv1 -level 3 -c:a flac -g 1 -y reference.mkv
mkv2xml < reference.mkv > reference.xml
xml ed -L --subnode /mkv2xml/Segment/Tracks/TrackEntry/Video/Colour -type elem -n Primaries -v 2 reference.xml
xml2mkv < reference.xml > COLOUR-PRIMARY-CLARITY.mkv