Is it safe to put boiling water in a margarine container? One concern is whether the plastic could soften, and another is whether the plastic coloring could leach into the water. (I presently use a ziploc bowl, but margarine bowls are lighter.)

Chris, I'd look on the bottom to see what number plastic it is, then go look it up online. You can see what the plastic made of, etc :-)

I'm not sure whether the number on the bottom tells the whole story. Both the ziploc container and a small yogurt container are number 5, but the ziploc is clear whereas the yogurt container is white. So is the white stuff an additive?

Maybe more to be concerned with is that the Ziploc container is rated for microwaves (so hence can take hot water) but the margarine container isn't rated for microwave use.
Somewhere I remember reading an article on what can be used...let me go find it!

Ok, here is one:
http://www.snopes.com/medical/toxins/cookplastic.asp
That should help some. I wouldn't worry about chemicals, but more that since it isn't rated for microwaving, your butter tub is more likely to get "soft" when heated with water. This in turn could cause it to dump food on you.
The Ziploc ones don't get soft though, so very well may be worth the tiny extra weight.

Sarah, thanks for the link! Chris