It's not dumb! The Schuyler Sisters one is mostly blue, but it does have a turquoise shade in some lights, I think. The Helpless one is greener, as it is a different dress. The Helpless one has like a... cape (there must be a professional name for this) as it is a winter dress (it's a winter ball!). The That would be enough is different as well, and I believe is the truest (?) blue. But yeah, blue is Eliza's colour.
(Sorry if this has weird grammar)