What are the reaction products in my dishwasher from a steel colander?
I am confident both are aluminum ; Steel would be magnetic, austenitic stainless is non-magnetic ,but normal relatively shiny or bright in consumer products. All steels are about 3 X more dense than aluminum if you could judge the density. Aluminum is subject to corrosion in solutions of salts, acids ,and bases. But I couldn't pick one of your components as the most likely cause. That is, I expect it is corrosion of aluminum you are seeing. I have had a very similar colander  but washed it by hand because it was fairly large to put in a dish washer.