Your great grandfather looked like many light skinned cape verdean do, my grandfather happened to look like him. I think him being called white in CV would have more to do with his physical appearance than what anyone actually thought his race was. This is assuming he was born and raised and his lineage is from Cape Verd. For example a Portuguese governor moving to islands would be called white on a racial level. Cape Verdeans often label each white, black, morena, mulatta, preta, etc on physical appearance rather than racial classification.