Hi everybody!

The problem is that every dark skinned NPC I make appears white in game

Any help will be appreciated

It's an issue that was introduced in Fallout 3's 1.5 patch that was never fixed. You'll need to change your plugin to a master file for the skin tone of your NPCs' bodies to match their faces. You can make the change with Wrye Flash, TESsnip (part of FOMM), or FONVEdit. You cannot directly edit master files in the GECK, so I wouldn't make the change until you're ready to release your mod.

By chance, do you know any mean to make black people appear black in the geck ?
In game it's OK, but as they are white in the geck it makes it difficult to adjust the skintone acurately and then in the game they are often orange or purple but not black.

The programs I mentioned allow you to create a copy of your mod, so the best I can think of is to make a master copy, take a screenshot of the NPC in the GECK from the master, then load the plugin and make whatever adjustments you need. As a master file, any skin tone changes you make to the head should apply to the body as well.