If you’re not familiar with Oes Gafr Eto the idea is that you’re singing about different coloured goats, starting with a white one. Each time you sing the chorus you add a new coloured goat, but you then cycle back through the previous colours getting faster and faster. For example:
a white goat;
a black goat, a white goat;
a red goat, a black goat, a white goat, and so on, going backwards through the colours
While you’ll see various sequences of common colours - white, black, red, blue, etc - this isn’t set in stone, so don’t be shy about adding your own colour choices. I can tell you that ‘piws’ gives your mouth muscles quite a workout though - try singing “finbiws” really quickly lots of times!
Here’s one example with the English translation: