can I join a college which has posted happy diwali (wishing Hindus on their festival)on on there Facebook page.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The fact that the college has posted congratulations on its Facebook page is not an impediment that prevents the permissibility of studying what is permissible in the college and joining it.

Merely studying in it is not an aid or approval of such prohibitions.

We advise you to avoid whispers and ignore them, and not pay any attention to them.

Allah knows best.