Then they get shocked when they see Pakistanis treat their slighty-darker-than-them South Indian cousins with human decency.
This is an exaggeration. In Pakistan, people marry all sorts. I have seen plenty of people marry darker people of both sexes. Complexion is just one aspect among many.
Among Punjabis maybe it is a little more due to high prominence of light skinned people among us too.
Being light skinned myself, people do comment on my skin color,, to which I say,, it is irrelevant and it is iman which is most important. That is something which I, and many others, believe firmly,
Ideal of beauty is unique among Pakistanis and generally favored lighter complexions (along with long nose, big eyes, light eyes/hair, etc.,) but however it is not the only parameter. We never mistreat dark people as a whole.
Also we don't necessarily consider White Europeans as beautiful. Pakistani concept of beauty standards is very unique to our country. Only comparable ethnic groups are Iran and Afghanistan.
Beauty standards of Pukhtoons, Kashmiris, Baloch, Sindhis, and Urdu speakers are also different, and not necessarily tied to color.
Actually being light-skinned Pukhtoon man is unfavorable among Pukhtoons and will make one the target of many jokes.