Bleeding hearts undersides are usually a reddish/silver. It may be that your two that are showing more color are more mature than the rest. Its also possible that they are female as well. The dorsal fin on a male is longer than the females and females are slighter larger and rounder than the males.
Its hard to tell the red in those photos, but they look normal to me.Are they acting any differently then the others?
its hard to tell using the dorsal fin because most of their fins are nipped off. THat's how I got them.
I didnt read your aquarium info... but that is an awful lot of fish to add to a tank that is only four days old. I really hope that you are doing daily water changes and keeping an eye on your readings. You have some wonderful fish in that tank and I would hate to see you start to loose them. :-\
My bleeding hearts change color pretty frequently. Same with my black skirts. When they're sleeping, their color fades to almost nothing. Freaked me out the first time I turned on the light in the middle of the night. Thought they were all dying. Then they woke up, and their color came back. I did notice that my bleeding hearts became more colorful in general when I added a few more to their school, but they're always changing. I wouldn't be too concerned about color changes unless they fade and look stressed for an extended period of time.