How long does mixed hair colourant last?

[Gingefringe]

I coloured my hair this morning with L'oreal Casting (semi permanent) hair colour but it hasn't covered all the grey at the roots. There's still a bit of the mixture left - will it be ok to use this evening or tomorrow morning or will it have gone off?

Ta

I thought it was dangerous to keep it. Doesn't the chemical reaction when you mix the two together keep happening and cause an explosion?
Or is that an urban myth?