If it helps at all, I’m able to reproduce this issue: my Oppo 203 plays the subtitles as gold, while my Vero 4K+ tends to render them as grey. A good example is at about 1:37:15, on the line “I kiss your hand, Don Tommasino.”
Playing it on my Amazon Fire TV Stick 4K Max, the subtitles also come out grey; so I would tentatively conclude that it’s a Kodi issue rather than an OSMC one.