I need to get hold of some silicone sealant for using on part
of the bathroom floor, but the snag is that aesthetically it
needs to be grey.

You're sure you're not supposed to be using grout?

http://www.decoratingdirect.co.uk/viewprod/b/BITHA6/

Might work. Claims to be marine, but is high modulus acetoxy (vinegar
smelling), so I suspect will work as a general purpose bathroom sealant.

http://www.decoratingdirect.co.uk/viewprod/s/SIRPF15LM/

Claims is a polyurethane building sealant, but claims suitability with
"flooring".

No guarantees that either product is what you want.

Christian.