The "agent" involved may affect the sound quality. If the agent is a solid material it will always have a texture (think of a a super super fine sandpaper) , it will cause the surface to be rougher than the surface if the colouring agent used is a dye. The solid particles can only be ground so small. The dyes being fluid provide a smoother surface. The Quiex II vinyl pressings that were used in Japan by JVC and popularized by MFSL use a dye to color the vinyl compound. These discs that use dyes are the translucent type. The solid agents make the vinyl opaque not translucent. It's my opinion that the translucent red vinyl on Canadian Rush's Hemispheres pressings sounds better than the black vinyl version. I am aware that they have different matrix data but in a few of the comparisons that I have made to demonstrate this indicate it is not a pressing to pressing variable.