Color map: ?Select color range' function
Alternatively, you can duplicate the original layer, and use the ?Select color range' function.
Color map: paste selection
Copy and paste your selection onto the base layer you painted earlier and adjust the Hue and Saturation to blend it in.
Blending in your selection by adjusting the Hue/Saturation
Color map: using photographs
Continue using photographs to get patterns. These can be anything, from tea stains to concrete, to achieve the effect in the metal reference. Keep your changes on separate layers. You can also play with the Opacity and layer modes: Overlay, Soft Light, Multiply, and so on, to make the patterns blend in. Use Curves, Hue/Saturation to refine the effect.
Use photographs and adjust the layer modes; Opacity and Hue/Saturation
Color map: fixing gaps
After you copy and paste the patterns on, fix unnatural gaps in the texture by creating a Layer Mask. Mask out or paint over some of the patterns that have hard edges and seams.
Fixing problems in the patterns
Color map: scratches
Now, create a new layer, paint in the scratches with a custom brush and adjust the Opacity. The example below is painted with a 1 pixel standard hard-edged brush with the pressure turned on.
If you're satisfied with your texture now, we can move on. Save your PSD into a TIFF file for the next step.
Painting in the textured scratches
Tiling the textures
Click on Filter > Other > Offset, and check the Wrap Around option. If your tile is sized at 2048 for example, then offset it by 1024 on the horizontal and vertical axes. Now you have a tileable texture, but with ugly seams down the middle.
Tiling the texture
To fix the seams, use the Clone tool to clone other parts of the texture onto the seams. Be careful not to touch the edges though.
Fixing the seams in your texture