Is it laminate?

Wow! That’s terrible, most Stone worktops are sealed with a special sealant, you can usually barely see the join. It looks like the worktop hasn’t been measured correctly and is a couple of mm short? Is it a large kitchen company doing the fitting?
I’d raise this with your fitter ASAP, that’s completely unacceptable and needs putting right.

Yes it's laminate, sorry should have mentioned that

I asked if it was laminate as opposed to solid surface or quartz because they are finished differently.

It’s not good. The cut is not clean enough. It may have been cut by hand or a router. If they used a router they need lessons. It needs removing and re-doing

He used something called colorfill in between and I think that's cracked? Or would it only crack if the worktop is not fitted well?

Not an acceptable finish. There was a long thread on here from another user. She eventually got the company back in to redo the whole thing.