If I wanted to ice a dummy cake a month and a half in advance (along with 3 others) and airbrush it, will the colour keep for when it comes to baking and airbrushing the edible cake?
Not completely clear about what you mean, but are you asking if you airbrush 3 dummy tiers now will they match an edible layer airbrushed in a month and a half from now ie mid May? ie you are making a cake that is 4 tiers in total?
If that is so my answer would be probably not. If somebody was paying me to make a 4 tiered cake I wouldn't risk it as even a subtle difference in colour would be very obvious. You could however, fondant the dummy tiers now and then air brush all 4 cakes in mid May after you have made the edible tier.
If that is not what you mean, apologies.