It's difficult to comment too much without knowing details of the individual concerned as those with dementia are all individuals so what affects one person may well not affect another. I suppose if someone didn't want to take part in an activity, or didn't want to mix with others in a group activity, they may feel unhappy. Or the subject of the painting might affect them if it brought back unpleasant memories, but as everyone is different it's really difficult to answer such a wide ranging question. Do you have a specific example you can tell us about? Was someone you know affected negatively as a result of a painting activity? What happened?