I have also learned to soak my fabrics in hot water before cutting them - like colors together - and to watch the water!
If the water turns color- the offending fabric gets soaked some more - until the water rinses out clear. Some fabrics have required several changes of water - and an occasional one gets thrown because I can't get it to stop coloring the water.
One of the first quilts I made has dark burgundy sashing - which I washed once in tepid (barely warm) water before cutting it - and when I washed the finished quilt, it did bleed a little bit. (Washed in tepid water - short gentle cycle - HOWEVER - I did not get it in the dryer right away - it set in the laundry basket for an hour or so before I dried it)