We are thinking of using grey plaques to signify people who have done important work within the government and white plaques for those who have done good works in the community. Lastly, our green plaques we think will be very popular, these will be for painters and sculptors, leaving our yellow ones for writers, actors and other people of note
It's definition 2.1 in Oxford:
Remain to be used or dealt with.
‘we've even got one of the Christmas puddings left over from last year’
Unless yellow plaques already exist and are used for more than just "writers, actors and other people of note" (that is to say, they are used for government figures, community figures, painters and sculptors as well), it's a little bit of an odd construction. However, what it ultimately means is that, the other colours being assigned to their various groups, everyone else (including actors and writers) gets to have the yellow plaques. It would also be a very understandable way of putting it if the writer/speaker had already introduced the idea that there would be these four colours, including what the colours are, before explaining what groups each one is for.