Yeah I read the chapter. Shunsui said Grey but when he attacked Stark it felt like a shallow attack since Shunsui wasn't wearing Grey. Stark did recieve damage it just wasn't a effective attack at all.
Starrk: Tch… // (The wound is shallow......?! // That felt like a blow strong enough to take my whole arm off...)
Starrk: ("Irooni"... // "Grey"... // My arm is grey...... // There is no grey on his body......)
Shunsui: What's wrong?
Now if you call a color that you are wearing as well. Your attack will be more powerful. Shunsui said Grey as a hint to Stark that it's not the coloring the person you are attacking is wearing but what you are wearing. Since calling a color you aren't wearing is a weak attack. Stark called white since he is wearing white not becuase Shunsui was wearing white. Shunsui commented that he was impressed that Stark count on that quick from just noticing from the observation from what I quoted above.
Basically the effectiveness is based on what you are wearing. Not what your opponent is wearing. Since Shunsui is wearing all black, now that he called out black his attacks will be extremely effective.
! Wondering, does this only apply to clothing? Could you techinically fight naked and say peach/beige and do a ton of damage? Or by only having clothes on does damage, wouldn't being naked negate the effect of the increased (and decreased) damage potential