Even so, when wet sanded with 1500 grit, the paint will leak. In fact, this is the case with all the paint I used including epoxy enamel and acrylic enamel. After cutting throught the top gloss coats of the enamel, the wet sanding evidently cuts into the colour pigments and it will leak color into the sandpaper and water bowl. It just makes sense. If there is no color in the water, then one isn't sanding color paint pigments but rather clear coat layers.
The color you see on the sandpaper is from the removal of paint as you sand the surface. It "powders." Splashing it around in the bucket, washes the sandpaper clean. Repeat the process. Water turns the color of the paint. Eventually change the water.
Squeegee the painted surface, as you sand, from time to time, shows the highs and lows. I.e. "orange peel." The sanding eliminates this. Goal is to get it smooth.