I am preparing for a wedding that will have an ocean theme and the cake will have shells (the topper is a light house). I want them to be as lifelike as possible and am molding the shells out of fondant and they are coming out wonderfully..... so far. I want to color them as lifelike as possible, too so I have even purchased some real shells to try to mimmic. However, I have only tried to color a couple and it is not proving to be as easy as I thought.

I have "sand" "copper" "coral" and other sea colors, but could use some advice from some pros. I've tried using some small paint brushes to dust on the colors in striped patterns, but I've only been 50% successful. I don't want to waste any of the shells that I have already molded.

Anyone out there have any ideas?

From your last paragraph you must be dry dusting w/petal dust. That leaves a very light bit of color on whatever. If you want a more intense color, add another coat of dust OR mix the dust w/vodka (or other clear alcohol) and paint it on.
Maybe you need a slightly bigger brush since you say you are using a small one. It should have very soft bristles and more of them than not....hard to describe. You don't want a pencil-point thin brush is what I'm getting at.....a little more like a fan is better. Just not sure what you mean when say 'small'

If you have an airbrush, that's the way to do it but if not I would use your food colors. You can mix them with extract or vodka to dilute.

I don't have an air brush. The brush i used was a soft, watercolor brush. Abt the size of the cheap ones kids use, but better quality and softer bristles, from a good set that I have. I was dry dusting. I tried the painting technique but I was not as happy. I've seen the affect that I am looking for, I just can't seem to copy it.

I will try the fan shaped brush. Thank you for the advice.