It happens from time to time and is not unusual in zinnias. Has nothing to do with it being eaten by slugs if that is what you are thinking.

Isn't that a Dahlia ?

Yes and it happens to Dahlias too. Sometimes the new colour is a good variation and worth propagating from cuttings.

... And you might end up with a Tri-colour.

If the original color was the darker one, the lighter, speckled parts might be a sign that the plant was infected with a virus. If the original color was the speckles, those are frequently a "chimaera", with cells from two different cultivars mixed together in their growing points. Occasionally--especially after severe injury--chimaeras will sort themselves out, with some branches suddenly bearing solid color flowers from one kind of cells only. That happens when a plant with variegated leaves suddenly produces shoots with all green or all white leaves.