What if it can't?

It is not hard to produce data for this problem so that there is no solution. For instance, if the total number of tubes is less than the number of colors; besides that, a number of empty tubes must be necessary. On the other hand, if there are as many empty tubes as colors, the solution is trivial, except fo some special configurations.

I don't know off hand how to detect that there is no solution. Supposing somebody does find out, what should be returned?