I love the look of the red cedar, I think it goes with any color. Stay away from natural or greys if you have a terricota styly brick. I think it is important to blend the tub with its surrounding opposed to making it stick out like a soar thumb.
As for shell, I prefer lighter colors, one that reflects the lights alot more. I prefer textured shells however, those pearl styly tubs are beautiful. I just heard they scratch easy.
If you go with the cedar style color I would go with the traditional red top cover. Keep the colors somewhat similar to the natural colors in the stone, from there you can accent it with some nice landscapeing. I can already see it.