One trick that I use to get a more realistic faded paint look is to realize that:
(a) Natural fading is never uniform - it's always patchy - more faded in some areas and less faded in others.
(b) Where the paint has faded, the viewer sees more of the underlying brick color and texture.
There is a simple way to simulate both of these effects. I placed a second copy of your image on its own layer and selected the sign area. I then sampled the color of the bricks not covered by the sign, and used the paint bucket tool to flood fill the area of the sign with that color (ie, color blending mode, so you get the new hue and saturation, but the original luminosity pattern). I also applied a light texture (sandstone) to this layer.
I then used the "render / clouds" tool to fill the mask for the "completely faded" layer with a random pattern to simulate the non uniform, patchy character of natural fading. If you don't like the patchiness that you get from the "clouds" tool, you can just paint in some random areas on the layer mask.
You can compare the result of the above to the original by clicking the static thumbnail below to see the animated GIF. View it low magnification to see the patchy aspect of the fading, as well as at 1:1 magnification to see things like there is less brick texture where paint is "covering" it.
Just a thought. HTH,
Tom
PS - I thought that the shadowing on the man was too extreme for good reproduction and that he was a bit OOF, so I tweaked these aspects as well.
