Translation:Red and white are the colours of England's flag.
It's asking me to say "the colors" when I don't see an "y" before "lliw", plus I thought "colors" was "lliwau". This sentence is very strange, I've had to report about four different answers now.
You have the construct state here ("lliw baner Lloegr" - the colour(s) of the flag of England), where this kind of possession makes the possessed thing definite by default.
Or another way of putting it is that only the final noun in a possessive series of nouns can take the definite article e.g. baner y wlad (the flag of the country) NEVER y baner y wlad; lliw baner y dref (the colour of the flag of the town) NEVER y lliw y faner y dref.
You're right that it should say "lliwiau". Well done for reporting.
If you've got "A is/are B", where "B" is something indefinite, you can use either mae or ydy, the difference being that an ydy sentence is emphatic:
Mae hi'n athrawes "She's a teacher"
Athrawes yw hi "She's a teacher"
If however "B" is definite, then you have to use the ydy:
Yr athrawes yw hi "She's the teacher"
Hi yw'r athrawes "She's the teacher"
never: *Mae hi'n yr athrawes - Makes no sense
So in this case, where "B" is something definite: "the colour of the flag of England", you have to use ydy too:
Coch a gwyn ydy lliw baner Lloegr "The colour of the English flag is red and white"
Does that make sense?