one of the problems manufacturers have with dyes is....we are all different- we all use different detergents, wash in different ways- they can not test & (combat) all of the differences- their fabric may be colorfast in the plant with what they use- but then you buy it & use something different---and oops---that one made it bleed-
if you check the end of a cotton bolt it always says to wash in cold water- many people use warm or hot-
there are many variables- they use the most dependable dyes possible and treat the fabrics as well as they can- but we get our hands on them & do things differently than they do---they have no control over that-
if you are using dark (saturated) dyed fabrics it is always a good idea to prewash them to avoid unpleasant issues in a finished quilt- or at least test-
and remember if you are giving (or selling) the quilt the next person may not use what you use- or do it your way---i made my mom a quilt years ago---she loves- i did pre-wash (it has alot of purple batiks in it) nothing was showing any bleeding when i pieced the quilt- the detergent she uses is much more (harsh) than what i use---all of the white in that quilt (which is alot) is now lavender & deeper purple---she still loves it- it breaks my heart everytime i see it---she didn't even realize it---when i saw it and said---what the heck happened to your quilt???she didn't know what i was talking about---then i said---well - it was white when i gave it to you! she just said- well it is still beautiful & it didn't want to be white i guess...
that detergent she uses takes the color out of EVERYTHING - regardless of how old it is or how many times something has been washed & been stable.