I have black 50mm tiles in 6x6 mats and am going to use a charcoal coloured grout. The adhesive is white and I find I am spending a lot of time cleaning the gaps between tiles from adhesive that has extruded when the tiles are pressed onto the wall. Is this a problem with my adhesive application technique or is there an easy way of cleaning the gaps to make sure no white grout shows through?

I would always have a small paintbrush at hand, and brush the adhesive into the joints below the surface of the tiles......Gaz

As Gaz has said you need to clean as you go I use a Nail brush dipped in water does the trick. Also if you can for next time use a darker adhesive if you plan on using a darker grout. the opposite would apply for light coloured grouts so you would use a light coloured adhesive.