Firstly, don’t use the Bright/Contrast node to generate a color. You should only use it to brighten or enhance the contrast of an existing color or texture. Use an Input>RGB node instead, or just use the existing color swatch in the mix node.
Secondly, when using a Mix node, the bottom value is used when the mix value is 1. So using the alpha of your texture, the text and lines would have an alpha of 1, and the transparent background has an alpha of 0. You’ve set the bottom value of your Mix to be white, therefore it makes your markings white. Just swap the color inputs around and it will use the color from the texture instead.
Thirdly, the only time you should ever add shaders while maintaining any realism is when adding emission to a mesh object, or adding emission + scatter + absorption together in a volume. Adding 2 Principled Shaders, (or diffuse, glossy etc.) will make the object scatter twice as much light as it should. Use a mix shader instead.
That said, something like this will work much better.
Also, @CarlG, you know can set the image texture to Clip instead of Repeat, and it won’t extend past the 0-1 UV bounds, right? That node group is completely unnecessary.