Just to play devil's advocate here, are you saying Belgium, France and the UK are good because they had colonies, and Switzerland, Andorra and Monaco are evil because they didn't? Or did the goodness seep osmotically across the borders? I feel like I agree with the gist of what you're saying, but I want to express it differently.
Thucydides expressed the moral problem of conquering another polity in his celebrated Melian dialogue: liewise writers from Tacitius to Gibbon have articulated the hypocrisy of Roman conquest and colonisation ("to their rape and slaughter they give the name of Empire: they make a desert and call it peace") the question of right or wrong in conquest and colonialism is well understood from antiquity.
I don't think countries learned colonialism is evil for the experience of having colonies, I think wealth and social development (often derived from colonial ventures, or trade with countries with colonies) and the opportunity for moral maturity that freedom from want gives led to changes in public attitudes.