I was soaking some fabrics in hot water to shrink them and remove excess dye from -
There seems to be no "rule" for which colors or manufacturers/lines that seemed to color the water.
A Moda red put lots of color in the water - and dyed the selvage strip pink. That was the only red of five that colored the water.
Just because a fabric is supposedly a "good" one - that is no guarantee that it is completely color-fast.