Usually a medium gray is considered a neutral that affects the piece the least. in general a darker color will pop the colors and recede into the background fracturing you pieces less. a lighter color will fracture the pieces more, breaking up the background. I personally dont like light grouts. I am a big fan of black and charcoal grouts. for this, I would do a mid gray and if there are areas where you are uncomfortable with you can do a paint wash to change it. You could also consider not grouting. I cant see much detail here close, but your work looks FABULOUS and if you cant see background thru the spaces, dont grout.
I wish my first mosaic had looked this good! Its lovely and you should be proud of it.

Wow, that is amazing. That's your first mosaic?!! I am partial to a pale sand color, about the color of your foreground. I can't stand white grout, and black or charcoal might be too overpowering for your piece, so I would go with the pale brown.

Well what ever color you use you did a great job!!!!
I do not think I would do a black .

WOW that is really nice! Great job.

What a fantastic job you did!! I totally understand your reluctance to grout because it can really change the look of a mosaic, and this one looks so good as it is! I don't have enough experience to make a specific color recommendation, but you could consider using several different colors of grout (ie a blue for the water, lighter for the froth, etc) Or, as suggested above, do a light neutral grout, and color wash any areas that don't quite look right. Whatever you decide, I hope you'll post pictures of the final result :)

Wow, wow, wow wow. Amazing job! Sheesh, my first mosaic was a stepping stone and a mess! LOL Here's a question for you and/or the group: Do you have to grout it? If you like it the way it is and it's not going to be subjected to weather, could you just leave it?

I had the same idea about not grouting it but unfortunately that idea did not come until the surfboard was complete. I planned on grouting so I was a bit light on the glue. (I want this to last a loooong time) The other thing is I would have made the gaps between the tiles much smaller. Oh well, at least I know for next time. I still have not made a final decision but I will post some pictures after I do grout.

if you are going to keep this inside and want a shiny look,, cover it with envirotex lite epoxy resin

It is absolutely stunning! Well done!

You mentioned that you went light on the glue, so you feel that you have to grout. From what I understand, grout is not an adhesive. Maybe doing an epoxy resin as kraftkrazy suggested would give you more peace of mind? (Keep in mind, I'm a beginner, too. :D)

my vote goes right along with 'wackyweeder'...
I also love the black and charcoal grout look.
charcoal will give you a bit of forgiveness to go ahead and put a wash of something else over any area you don't like..and want to change a small area of color.
but from what I see, the charcoal will define the entire piece and in my personal opinion, the grout is what makes a mosaic look like a mosaic..meaning the color in between each individual piece, to define it, rather than a solid color, like a painting.
I also love the look of pieces being pressed into the base, like the thinset, to give it that 'memory jar' look..
I've always been a huge fan of that but your piece is much more refined than that.
and did you snag one of your kid's skateboards for this?
will they miss it or did you replace it?..lol