Julie- there are some great threads in the past about grumblers' shops, pics of design spaces, etc. Grey, black and beige seem pretty popular for the material behind the actual samples on the walls.
I used grey, b/c I got a good deal on a whole bunch of the material.
BUT, I have four different painted walls in my shop (one being a dark red)- I purposefully chose colors that ANY frame will look good on- and customers will bring the frame over and use my wall as a ref. for their own wall color, or something close. I did this crazy 4 color choice after I got tired of having the taupe be too bland, and the other color, a kind of green, making some frames hard to sell! I just like to hold things up off the design counter, and up against something for them to get a feel...
Grey countertops should work well, too.
No opinions on Velcro- I reinforce with glue, staples or more Velcro - some sample sets are bad about the Velcro coming off, others, no problems?
Hope any of this long winded reply is helpful- good luck!