Latin Americans tend to have more melanin on their skin, the pigment that makes the skin darker. This migtht be because the climate was hot and sunny in ancient times and people depended on agriculture, which required working in the sun for many hours at a time. The skin had to adapt and therefore produced more melanin to protect itself from the suns rays. It's not really necessary anymore, but it's carried over from ancient times. Kind of like a vestigial structure, I guess.
depends what they mixed with before. just because they are close does not mean anything. my parents are from Macedonia, and when i go there, i can tell apart the Greeks (it borders it) from the Serbians (borders that as well), because its a different group of people. the language is similar, i understand Serbian, even though i never took classes for it. same with those 2 countries, they have a different past, causes them to look different.
Costa Rica and Puerto Rico are located in different areas. Costa Rica is in the small strip of land in central America below Mexico, bordering the pacific, and Puerto Rico is made up of islands in the Caribbean, so obviously there is going to be some variation in the natives.
Obviously you've never been to Costa Rica or P. R.,otherwise you would know that just as in most countries there are people of wide range of skintones in each. I have friends from both who are whiter skinned than I am(I'm Northern European Blonde)w/Blue,blue& Green eye colour and; also very darkskinned. Just Depends on mix of ancestors. Costa Rica has a LOT of German,Austrian and Eastern European Jewish mix within the population as do many of Central and South American Countries. By the way; Just What is Bannana tone skin???