How long must you wait before reapplying a different haircolor?

It depends on the condition your hair is in and the kind of process used on the hair. A process that lightens the color of the hair will be harsher to the hair than one that simply deposits color. If your hair hasn't been damaged by the color process you should be able to recolor the hair as soon as you desire. But bear in mind that you want to avoid over stressing the hair with too much lightening or with lightening haircolor processes over permanent waving the hair. If, for example, your salon lightened your hair and you preferred that the color be darker than the result, you can safely add an additional deposit-only color to darken the hair again the same day. However, if the salon lightened your hair and you want it lighter still, you should consider waiting at least a week (with conditioning treatments in between) before further lightening the hair. It is always preferrable to seek professional assistance in any dramatic color change or if you feel a salon has messed up your hair. It is courtesy to offer the responsible salon a chance to make things right (accidents can happen to anyone) but it is understandable that sometimes this isn't an option. However, make sure you are prepared to accept the consequences if you opt to take matters into your own hands.