Grape is a noun, and flavour is a noun. When you put two nouns together to make a compound noun grape flavour, only the head word (normally the last noun) is ever pluralised. With grape flavour, I cannot think of any situation where flavour would not be the head word. So, grapes flavour is never used. grape flavours could be used, and this would describe a number of different flavours associated with grapes, for example:
There are many different chemicals that are used for synthetic grape flavours
Regardless of whether you are talking about one grape, many grapes or the general concept of grapes, grape in grape flavour is always singular.
I like grape flavour bubble gum
In this wine, the grape flavour is very strong
For more information about situations where the head word is not the last, see this article about compound nouns.
Note that
grapes is not a collective noun: it is a simple plural of
grape. A collective noun is a singular word used to describe a group of things, for example
team,
population,
flock (of sheep),
shoal (of fish). This article gives more information about
collective nouns.
Note also that flavor is the US spelling, and flavour is the UK spelling.