Teo KiKi takes us through some of the key aspects in painting a metal tileable texture in Photoshop.
The main point to remember when creating your tileable texture is to make the appearance as generic as possible, as it will be tiled over a model surface.
Color map: Background
We'll begin with a color map. First, reference your callout sheet and analyze it. Then, as we'll be painting metal, we need to start with a black background.
Starting out with a black background
Color map: adding color
Pick the complementary colors on your reference image and using a Cloudy brush, paint the basic color variations into the image. Here, this example uses red and blue as most types of painted metal have a very powdery look.
Adding color variation with a Cloudy brush
Color map: patterns
When you are done adding color to the base, you can start extracting patterns from reference photos.
Using reference images to add texture
Color map: contrast
Using the RGB channel, look for the channel with the highest contrast on the image. Duplicate the channel and adjust the curve for more contrast.
Add a starker contrast to the image
Color map: select patterns
Now Ctrl+click the thumbnail of that channel to select the patterns.