The first thing to remember that culture develop through Larmarkism not Darwinism. That through processes like syncretism cultures develop over time. Consider each cultures we know about as a descriptive list. From time to time not only culture add and subtract from their own list they grab stuff from neighboring culture list. Mind how this happens varies and it is not always voluntary (i.e. conquest).
The approach I found to be the most gamable is to go through what I know about real world cultures and make up descriptive lists of the one I am interested in. This includes customs, rituals, beliefs, religion, government, and so on. Whatever you think is you can portray in a roleplaying session.
Then for your setting you take these lists and make unique combination that represent your in-game cultures. For example my Ghinorians in the Majestic Wilderlands are a hybrid of Hebrews, Western Medieval, and Roman culture. Basically the hebrews found my setting's equivalent of the Roman Empire and later fragmented into medieval states. But they are not entirely hebrew as I borrowed elements from India, China, and Japan. The first two for the Imperial phase of their history and the latter for one of the successor kingdoms.
Remember to look at non-western cultures from India, China, Japan, Southeast Asia, and so on.
I strongly suggest you do not deal with skin color in your setting. I also refer to things like "You see a Ghinorian, a Tharian, and an Elessarin talking together). Rather than a black skinned individual, a fair hair white skinned individual, and a freckled red haired individual talking together.
First whether we like or not race is a charged issue in western society. If you don't have to get into it don't.
Second physical descriptions are a burden on your players. Not only they have to remember what a Ghinorian is but also that they are dark-skinned. I cut them some slack and just jump to the label and go from there. In the end what important that they react to what the culture is not play a guessing game. So when I say they they see a group of Thules walking down the street they suspect they may be dealing with Setites as their culture is noted for the worship of the Lawful evil god Set.
Remember that people are people. Human being have common considerations that cut across all cultures. For example the rules of hospitality are fairly standard. The few tribes/culture we know of that don't practice these rules are considered assholes by their neighbors and even by the anthropologists that study them.
Non-humans in contrast may have different consideration. The trick here to make up the list what all members of that race are concerned about. Then the various cultures of that race have a baseline from which they add to or vary from. For example my Dwarves have a low fertility rate with makes their culture very clannish and obsessed with protecting the females and young compared to the human norm.
Whatever you come up with remember to keep it playable and understandable. It is not always the same as realistic and accurate.