setAdditional color sets an additional color, i.e. it adds the color. In jPCT, there is no such thing as an object without a texture. Even objects without an actual texture assignment have a texture, in this case a white one. Adding any other color to white doesn't change anything, because it won't get whiter than white. A light source's color gets multiplied with the texture color, i.e. you can't light up an object textured with in pure black, because 0*something is still zero. That's the way in which the default OpenGL lighting pipeline works. If you want something different, you have to use shaders...or somehow adapt to it's behaviour. In your case, I suggest to texture the object with a color that is almost black. You can use the Texture(width, height, color) constructor for that. Just create a small 8*8 texture with maybe 10,10,10 as a color and see if that gives you the desired results.
