Take in OpenBRF a look at the shader (check the material of the mesh and there the shader entry), perhaps it's using a specific shader just for that banner. Switch it then out with the normal one which is getting used by the other banners. Otherwise it might be that there is a hardcoded vertex colouring of the mesh.
Otherwise you might not be loading the mesh correctly, make sure it is getting called by the engine.
Perhaps tackle it a different way. To make sure that your new mesh (and thus texture) is loading fine, use any other mesh which looks clearly different than the banner (a weapon, helmet, etc.) and name it "tutorial_flag_yellow.44" (flag). Then check what happens in-game.