NB: Title should be MOST spells cannot be written!

My bard just went to Level 4 (vanilla BG1), and has an extra L1 and a new L2 spell slot. So I trot off to High Hedge to buy scrolls so I can learn spells to fill them.

But most of the scrolls are red or green (cannot be written to spell book), including basic ones like Sleep and Blind. My bard Int is only 13, but Protection from Evil (L1) and Mirror Image (L2) are available. What is happening here?

Are you talking about the colour of the actual picture on the scroll? If so, that has nothing to do with whether you can learn the spell or not. Rather, it indicates the function of spell - generally, damaging, debuffing (e.g. Sleep and Blind which you mentioned) and summoning spells are red, protective spells (e.g. Protection from Evil and Mirror Image) are blue and other spells are white (there may be some exceptions, I can't remember). Green pictures are for protection scrolls, i.e. the sort that aren't actually spells but can be used by any character and provide protection from a certain type of attack (not to be confused with scrolls of a wizard or priest spell of a protective sort).

If an item (including a scroll) is one that your character cannot use (usually because of their class) then its icon - the entire square, not just the picture on the scroll - will be tinted red when the item is shown in a shop screen with the character in question selected, or when it is in that character's inventory. (Similarly, unidentified magical items are tinted blue, though that should never be the case on a shop screen.)

Thanks, you're right! *facepalm* I tried this with one I found as loot, but must have had finger trouble..