Darren,
In general, as long as each coat of paint is
completely cured, you can paint any type of paint over another type without problems. Problems will occur if the underlying paint is not completely cured. There are many factors affecting your paint's cure time: How thickly the coats applied, the precise solvents in the paint, the relative humidity during drying, etc.
If you're really concerned and the area of AS bare metal is completely cured, you could overcoat this area with a coat of Future or clear gloss. This will place an additional layer between the blue you will be applying and the base silver to help prevent any reaction between the two paints. If you do this, allow at least 48 hours (depending on your application method and environment) before putting the blue overcoat on the model and you should be fine.
Enjoy your modeling...