WPF图像，改变像素的颜色

I have a problem. I need to fill (or repaint) some pixels at image, stored in Image control. This is a png image. I mean, that all black pixels should be filled with, for example, red color. How can I do this? I thought I can access directly to pixels and using XOR change special bits, but I don't know how to do this. Or maybe there is an easier way?I have a problem. I need to fill (or repaint) s