I have my hair coloured in permanant colour and I`m only 20 (so definately not going grey yet. Im not an expert but I think Semi-permanant colours will have lots of excess colour in the first few washes because the colour doesn`t "sink" into your hair as much just because of the fact that it isn`t permanant.
Semi-permanant is a better option for hair though as it conditions it, plus you don`t get roots cos the colour fades before.
If its semi-permanant the colour will fade because thats what it is designed to do, although it shouldn`t be rapid...It should last 3 months at least.
How is the colour now, has it faded at all?
If it does fade and your not happy you can always go back to the salon and give them what for.