4.2.2-4 May a legitimate buyer of a black-and-white photo convert this photo into a colour photo and justify this action with the principle of exhaustion?

No. Although the buyer became the owner of the photo, the conversion of the photo is an alteration pursuant to Art. 11 para. 2 CopA. Only the author may make alterations; it is the moral right of the author. This right is not affected by the principle of exhaustion.