Is this your first aquarium? Because that will change my suggestions quite a bit...
As far as sand goes, personally, I don't really like using it as a substrate. It is about impossible to clean and that means you need alot of bottom feeders to clean up the mess.
I like the gravel and it comes in about every color under the sun. Black will make your fishes color appear brighter.
Using the shells shouldn't be a problem at all, just make sure you rise them, rinse again, and really rinse them well again before putting them in the tank. Especially if you got them directly out of the ocean.
Some of the most colorful and popular freshwater fish are the livebearers (guppies, mollies, platties and swordtails) All these come in a galaxy of colors and you just have to pick your favorite ones. I have a 20 gal livebearer tank with black gravel, natural lookin fake plants and some pink and green accent stones and I think it looks great, but that's my opinion.
What fish to get also really depends on what size tank you have. I could recommend a stocking scheme for you if I knew the size of your tank and your experience level with aquariums. I love picking out fish!!!! Let me know and I'll get back to you
I'll even post some pics of the fish I'm talking about so you can see them before deciding.