I've installed new textures and they come up white! This is extremely frustrating so why are they doing it?
Definitely need more info to be sure what your problem is but if it's a scenery file you may need the Nova textures. Google for the Nova website. There are textures there that a lot of scenery designers use in their sceneries. The designer doesn't provide them so you have to get them from the Nova site.
Also, if it's a problem with trees your scenery may need Gerrish Grey's tree sets. That file should be here at SimV in the download section.
I've also run into a couple sceneries where the textures want to be placed in the default FS "Textures" folder instead of the textures folder inside the scenery folder. It's worth a try moving the textures over. Just make sure they don't overwrite one in the default textures folder. The file may have the same name but be completely different. Make sure you click "No" to any request to overwrite.
For aircraft it's a different matter. If the reflections checkbox fix mentioned in this thread doesn't work then you will need to post a link to the airplane you are having difficulty with.
Often however if you run into an add on aircraft texture that shows up white and it isn't because you don't have the reflections checkbox checked, it's because the texture file you are using is made for a different aircraft. There is sometimes more than one designer making an aircraft and even sometimes the same designer making two completely different versions that can't share textures.