My preferred method of cleaning lately has been to soak the parts in a warm solution of dishwashing powder. I've found that if I boil them then there's likely to be some colour change but if I only warm the water to a point where it's just getting uncomfortable to take the parts out with my fingers then they can stay there all day with no colour change. You said you'd rinsed off the sink cleaner before boiling in tap water and that's when you got the colour change so I can only guess there's something in tap water that can affect aluminium when boiled. Our tap water has a fluoride treatment, not to mention all the other minerals and such that's always in tap water.