These conditions are difficult on the painters and the paint. The good part is their painting aluminum siding not wood. This siding won't adsorb water like wood and will dry for painting in just a couple of hours, if the temperature is good.

The disturbing part is painting "right up until the rain started falling." This could cause some real problems; running paint, separating colorant, dented or cratered paint film to name a few. It would be nice if they would quite painting a little earlier but you can check the paint job for these potential problems and document what you see, take pictures of any found problem areas. If you find any unsightly areas point these out to the painters and the carpenter, these will have to be fixed before final payment.

You mention Sherwin Williams paint. Do you know the brand? Super Paint, Resilience or Duration? These are all very good paints but Resilience is designed for questionable wet weather. Any paint will have its limits. The best way to prepare yourself for possible problems is to call your nearest Sherwin Williams store and talk to the manager. It is likely they will send out a field sales rep to take a look at your home, the products being used and render any needed opinions. If the painters are using any of the good products offered by Sherwin Williams the sales rep will make sure they are applying everything correctly, Sherwin Williams offers product warranties and I'm sure they will prefer the paint be applied correctly.