We could deal with this by many solutions, one amongst is:
Bring an arbitray bottle/can of vingar of any odoriferous liquid, pour some of it onto one previously determined ball, tell your friend to taste it, now tell him to swap these balls as many times he want, then tell him wether each one taste like vinegar or not remotedly, your friend will get baffled when you always point on the different one without even get enough closer to smell it.
Another approach if the previous one is kinda deja-vu:
Let's bring a dog, ok forget it, a dog can smell from distance, a cat, hmm even cats are attracted to round things no matter they are, but well let's give it a chance, dip a ball of predefined color in fish soup, bring it few inches next to this cat, the cat will taste it and enjoy it, take it off from it then retry the same process, put it forward a cat's scope of view tens to few hundred feet, until enough far from it, the cat will arrive following the conjunctive view-taste phenomenon that has been insinuatingly impressed into its subconsciousness, always under our observation, we carry on the process but without sinking the ball in any soup, the cat intuitively comes into the ball, if ever it hasn't an appeal to play with it, it goes away disappointedly. Now if we call another cat, iterating the same process with it, until the point the ball is not merged with fish soup, instead of revealing the ball of same colour to the cat's field of view, we use the ball of different colour, the cat wouldn't be expected to apporach the ball, why? it's because the second ball doesn't bear a common conjunctive factor of view with the other ball, the cat isn't turned on. From this behavior, the achromatic person can understand just from events sensed from his surrounding, the imperative existence of another factor apart the ones he enjoys, which has influence on living beings and their reactions. so called colour.
If purposely we can break some of "zero knowledge proof" 's rules, we could approach the question in another way. This is contrarily not supposed to be a valid and conforming proof due to "conspiration" factor especially considering the "skeptical" nature of the person in question.
Calling out a random person occasionally passing by, blind-folding him, then we hand him the balls at many reprises with telling him to drop "the one he thinks special". The subject would, at some extent, drop the wrong ball. Always under the witnessing eye of the colour-blind person, we tell the one in experience to drop the special one without wearing the band, he would never miss. This can be same way expressed with that person, telling him to pick a special ball, then asking him to drop that ball without seeing it, at some round he would definitely say "Oh I missed the right one!". What makes him think he misses? Why haven't he had mistaken the special ball when the band was removed from his eye? what kind of this insensible factor which is acting upon that person's choices?
Such kind of questions and methods are used in real life by psychologists and social experts to map the patient to his abnormalities that he isn't aware of, without forcing him unquestionably to get convinced without self asking why. (this is a little off topic margin so my excuse)