Having restored several early sandcast engines, I can tell you that all those that I have worked on were indeed painted in an "aluminum colored" paint. Yes, the engine cases, cylinders, head, breather cover and cam tensioner housing are all painted in the same aluminum color. Some engines appear to have no paint on the oil pan, while others do. Markb noticed small shade differenced in the aluminum paint on a few parts of his very very early engine (100). Your engine should be painted in aluminum.
I have used two products to get the right color. The first is Duplicolor Engine Enamel Cast Aluminum 1650 in a spray can. You will need to prime the parts with Duplicolor Engine Enamel Primer as well of course. I bake my parts after painting them with Duplicolor Engine Enamel. 20 minutes on the grill outside at around 200F will cure the paint nicely upon cooling. This is a nice finish the is vary accurate and fairly durable.
On the last few engines I have built, I have used a different product: POR15 Engine Enamel in Aluminum. It does not require a primer and is sprayed on with a spray gun (you can brush it on, but I don't think you will like the results). Why did I switch? I find the quality of the paint to be significantly higher. I can control the flow of paint with my spray gun perfectly too. The color is perfect too.
In fact, I did a test...I sprayed two parts and let them dry for a week; one with duplicolor and one with por15. I then tried to scratch them. Both are strong surfaces, but the POR15 paint is much stronger and showed no scratches after several tries. The Duplicolor paint showed scratches quicker.
Hope this helps!