I disagree. Even the same mixture can have a wide variety based on how you do the pouring. In fact I doubt you could come close to reproducing the effects from pen to pen using the same mixture no matter how hard you try. So don't shy away from doing the same mixture. Resin is similar to wood. Even blanks from the same board will produce a unique pen.
I haven't made enough pens for it to matter, and even though the patterns, swirls, proportions, etc. can be different, the colors are still the same.
It may or may not make a difference to the recipient, but personally I rather make many different colored casts, rather than a bunch of the same colored casts. More variety for the occasional caster.
From the standpoint of selling, it might make sense to cast a large batch of the same combination, to save time and increase profit, since a seller doesn't care about how many pens are made from the specific color combination.
If you were to take two pen makers, have them use the same materials (wood/plastic) and put it on the same kit, with the same treatment (painting tube/hole, etc.) and give it the same shape, will anyone know who made which pen? The pen will only be different in that the pattern/swirl, or grain might be slightly different.
The recipients of my pens, never mentioned the swirl, only the colors. So how much of a difference does this make to an individual? Do they choose a pen based on the pattern, or the color?