What's the best sealant for coloured concrete coping stones?

[Bramco]

Hi,

 

The edges of our patio and a raised planting area are finished with coloured concrete coping stones.  

Unfortunately these give off a coloured dust if you sit on them which given they are grey is a bit of a nuisance.  Would be great if we could seal them somehow, so that they stop giving off the  dust. 

 

Has anyone done this before? And if so what did you use?   Otherwise, any ideas?