Now, is the soft mutation on du because there are two adjectives in a row?
The mutation is because nos is a feminine noun, and these cause mutation of their following adjectives however many there are.
Why do we say "nos da?"
It is an exception which goes back several hundred years. Similarly, wythnos diwetha (last week).
Aah, interesting! Thanks, have a lingot :)
It's the final s that has a tendency to prevent the d from becoming dd in these common phrases. (You will hear some speakers use wythnos ddwetha in their dialect however.)
What if you were to add a third adjective - it's a dark, black, cold night? Would you just string the third adjective on after thr first two (nos dywyll ddu [...]) with no punctuation?
The punctuation rules are the same as English here. You could have a string of adjectives with or without the commas in either language.
I've just noticed this sentence is wrong. It should be Mae hi'n noson dywyll ddu.