Here's two custom straps from Stewart at Heuerville and a tropic strap (bottom left) as some ideas. Google Heuerville to see the significant selection of colours he uses. He's obviously not the only one that does custom straps but gives some good ideas....and he's in the UK.
These are 20mm versions, but you can obviously get them in 18mm and tapered to whatever buckle size you want. I fit reedition Heuer buckles on when I put them on my watches.