The backround node you are using is in Color mode while your foreground node is in Grayscale, that is why (the yellow line means this is a color node, gray line means this is a grayscale node. You can't blend between color and grayscale - they both have to be either color or grayscale.
You have to options: either convert the grayscale node to a color node by connecting it to a Gradient map node (for example) or convert the color node to a grayscale using grayscale node (or also use the gradient map - it has a switch).