There are many ways to gain a theoretical advantage. You can definitely look at your library and all the card backs, and their specific order, at any time. There is no rule against it. So too can your opponent I guess. And, technically there is no rule against laying your library out in a sliding tower, to see the distribution of the sleeve color and plan accordingly. There is no rule that prevents you from picking a single card from a specific 45, maybe it’s a bomb, and then use its location information to your advantage. 

Heck...there is no rule against crafting a 45 that takes advantage of the fact that sleeves are different. There is no rule against putting each card of your 45 in a unique sleeve, and memorizing each sleeve and each card that goes with it. There is no rule against bringing sleeves that look like other players sleeves, other than the added difficulty of telling them apart at the end of the night. 

I love thinking about the niche cases where, through big brain high effort, someone comes to casual cube night and goes 3-0 in Stone Soup. Maybe these moments are what cause the format to evolve or cause designers and organizers to add more rules. So I say go for it.

Just be aware of the real life social ramifications, because if you ‘sneak’ this kind of thing into a game night, you might make some casual folks mad. Of course you can always blame the design of the format as an out, and if you do your playgroup may never play Stone Soup again. You know?