Your problem is equivalent to maximum matching. In an optimal coloring, each color class is connected. Choosing one edge from each color class, we get a matching. This shows that the maximum matching is at least the antichromatic number. In the other direction, take any maximal matching, and color each of the pairs using a different color. Every other vertex is adjacent to some colored vertex (otherwise the matching isn't maximal), so you can extend this to an antichromatic coloring, showing that the antichromatic number is at least the size of a maximum matching.