I partially disagree with SIMPA (sorry, man). If you have made your own experiences with it, it is simple to create flags. If you are new to this topic, it can be very confusing.
What you should do is look in your mods CIV4PlayerColorInfos.xml. There should be blue defined for the civ (in this case America), probably as secondary color. Change it to white. I bet this will solve your problem.
You may also check if the flag image contains an alpha channel. This sometimes causes problems. Some machines like it only with alpha, others only without. So far I didn't played civ4 on a machine that worked with both. If publishing a mod, you should provide a second version of your flag that users can use to replace the default one if they get troubles. For developing a mod, you have to find out what sort of machine yours is by testing. If the flag completely appears white in the game, then this is a sign that your machine doesn't like the alpha channel "setting" of that flag. To solve this, you can use DXTBMP, dds converter (you can find both on civfanatics), GIMP (with extra plugin) or Photoshop (also with plugin). Since Photoshop isn't free, I recommend GIMP if you don't already own PS. The other two programms are also free, but they are either buggy or unhandy. Unfortunately I don't use GIMP by my own, so I can't help you with this.
The last thing that causes problems, is the <bWhiteFlag> entry in CIV4ArtDefines_Civilization.xml. If a flag doesn't use the civs colour (your case, I think), that entries value should be 1. These flags usually contain more then 2 colours. If a flag uses the civs color (the color of some parts of the flag, usually an emblem or symbol, appear in the civs primary color), the entries value should be 0. This kind of flag is called a bicolor flag. If this setting is wrong, your flag should be completely in the civs color (not sure if primary or secondary).
Hope that helped.