What could I use on pebbles or shells to reveal their natural colors?

Clear art sealer. You can buy it in little bottles at craft stores.

If you wash and use a sealant that should bring out the colors in them. Rustoleum makes a great clear sealant in matte, satin and semi-gloss.

Make sure the are clean and dry,spray them with a outdoor clear uv protectant spray from Rustoleum

matt mod podge works beautifully. i have used it for years to do just what you are wanting to do.

Americana craft varnish shinny

First check out the videos on YouTube for how to. A friend who lives in sunny Florida uses Muriatic Acid to clean. Mix 3/4 C. water with 1/4 C. Muriatic Acid. Measure water first in large glass container. Slowly add the Muriatic Acid and mix carefully. Dip shells with tongs one at a time for about 3 seconds each. Rinse by dipping in clear water immediately. Let dry on a paper towel. This will bring out the beautiful colors they had in the ocean. Safty glasses, rubber gloves, long handled tongs, face mask and small brush along with the glass container. There are videos on You Tube that show how to do this project...outside in well ventilated area

. Do not use near stainless steel as it will cause discoloration. Baking soda is a must to neutralize spills or accidental contact with skin. Muriatic acid is available at Home Depot or Lowe's. Please exercise safety and do not do this project around children or pets.

have you tried clear nail polish?