If this is plural, I would have expected the ending in the adjective to end with -i instead of the -e. "Jasne" is for plural feminine and neuter, isn't it?
Yes. the masculine personal is "jaśni", but it's hard to find a natural use for it.
kolor is masculine so jasne, here, is an exception? Or is "jasne" typically used for all masculine plural as well as feminine plural and neuter plural?
'jasne' is either neuter singular or 'not masculine-personal plural'.
And as you have plural 'colours' there, and colours aren't masculine people nor people at all, that's 'jasne'.
A singular bright colour is of course "jasny kolor".
It can in English, but not in Polish.
'jasny' is mostly either 'bright' (bright colour, it's bright in this room) or 'clear' like in "Is everything clear?"
The ending for Kolory is hard for me to hear on this particular example. Sounds more like an en or em sound and less like y? Maybe it is the lifting terminal indicating the question that pitched the y sound for me?