What skin color did adam and chava have? I learned that if all people indeed came from them at least one of them had to have a dark skin color.
R. Abraham Viterbo deals with this in Ma'amar Chamishi of Sefer Emunat Chachamim (bottom of page 40a). He begins by mentioning the idea that dark skin was a result of Noah's curse to Canaan. In that case, he says, we would have to assume that all the physiological differences between people of different skin colors occurred at the time of Noah's curse, and not due to the heat of the lands they lived in. He supports this by noting that when northern people with light skin move to the southern locations where it is hot and the people have dark skin, their skin does not turn dark – nor does the skin of their descendants change even after several hundred years.
However, he then argues the reverse that it is indeed because of the heat that their skin became dark. In response to the aforementioned objection, he explains that skin color is not a binary choice; rather there is a continuum ranging from very light to very dark. Skin color proceeds along the continuum from one country to the next, and changes very slowly over hundreds of years as the heat enters their bodies. If we accept this approach then Adam and Eve would have had light skin and dark skin evolved over a very long period of time.
We don't know. Racial traits evolved later, hence the premise of the question is called into question:
Tiferes Yisrael writes on Sanhedrin 4:5
ומלך מלכי המלכים הקדוש ברוך הוא טבע כל אדם בחותמו של אדם הראשון ואין אחד מהן דומה לחבירו: לא לבד במראה שיש שהם שחורים כפחם ככושי ונעגער והאטענטאטען, ויש לבנים כשלג, כגון זאמעידען ואלבאנוס, ויש אדומים ביותר כגון תושבי אמעריקא, ועוד הרבה מצבעים שונים, די"ל דע"י שהם זה אלפים שנה באקלים ההוא נשתנה טבעם וצבע עורם ונקראו מחז"ל כושי לווקן גיחור [כברכות נ"ח ב'], אלא אפי' הנולדים באקלים אחד אין אחד דומה בתוארו לחבירו:
(Mishnah: A human makes one mold, and all the coins produced are the same, but God minted one human, and none are just like another): Not just in appearance, that some are black as coal, like the Ethiopian, Negro, and Hottentot; some are white as snow, like the Samoyeds and albinos; and some are really red, like the dwellers of America; and many other colors -- as we could say that because they were in those climates for thousands of years, their innate skin color changed ... rather, the Mishna means that even those born in the same climate, none look alike.