What should I do to distinguish an item which has the same color with its background?

I'm making a game where some discs/circles moves around in a tiled map.

Each disc/tile have a color, and sometime the color of the disc and the tile are identical.

It would be hard for the user to know there's a disc lying on that tile, so I'm thinking of a way to make the disc reasonably visible. So far I've thought of 3 ways:

1. Darken/lighten the disc


2. Give the disc a dark outline


3. Give the disc a shadow

This is how I'm doing it currently, a combination of No.1 & 2.

But I'm not satisfied with it. Method 1 make it hard to recognize the true color of the disc when I have 8 or 9 different colors in the game. And method 2 and 3 both feel old.

So does anybody have experience with this and can help me out? How would you solve this problem?