Ok so as you're using a MixRGB, you can indeed mix the 2 Image Texture nodes into the MixRGB, then you need to plug the MixRGB into a Diffuse, then the Diffuse into an Output:
MixRGB is used to mix 2 colors and/or images, but you need to tell Blender what kind of shader it is supposed to be, between Diffuse (which will be affected by light and shadows) and Emission (which emits light). In your case I guess you need Diffuse.
Another equal solution would be to use the Mix Shader, in that case you need this setup, as Mix Shader doesn't directly accept images or colors:
