they are colour coded and that would be great if Larian hadn't forgot to tell us what the colours mean
Are you sure they're color coded and the colors mean anything? They're definitely NOT based on school.
Trying to find a pattern...
Green = Necrotic, Acid, Poison - but NO because Ray of Enfeeblement (necrotic) scroll is pink!
Blue = Ice & Electric - but NO because Sleet Storm is red!
Light Blue = Utility? (Feather Fall, Dimension Door, Fly, Jump, Speak with Dead) but NO, Darkvision is black
Orange = Fire - but NO because Cloud of Daggers and Spike Growth aren't fire and flaming sphere's scroll is gold!
Gold = Buffs (Mage Armor, Bless) - but NO because Guiding Bolt, Grease, and Moonbeam are gold too!)
Teal = Healing & restoration
Purple = Thunder (all 2 of them)
Pink = ??? Charms maybe? (nope, Fog Cloud is not a charm)
Black = Spells with the word Dark in their name. Again, all 2 of them.
So maybe there was some general guidance but they were allowed to ultimately do whatever for scrolls? Because there's exceptions to every group I try to create.
But yes to answer the OP question, Wizards can learn almost every spell from a scroll.
I wonder, if your WIZARD speaks to the merchant and you right-click the scroll in the merchant's inventory, does it indicate whether or not the spell is already known? Once you have the spell in YOUR inventory, the wizard can right click and if the "learn spell" option is available, that means you don't already know it and can learn it. I'm not sure if there's an indicator while the scroll is still in the merchant's inventory.