If you keep in mind that all specularity is simply added to the diffuse (as a simple mathematical addition) we start understanding how this part of the render works and we have a little more control over it.
Think about the colors in a shader as numbers that go from 0 to 1 in the RGB scale (yeah, not 0 to 255). So 0-0-0 would be black, 1-1-1 would be white, 0.5-0.5-0.5 a mid gray, 1-0-0 red, 0-1-0 green, 0-0-1 blue, and so on. If you know the values of your diffuse color, and you know the values of your specular color, you just need to add these three numbers individually to see the result of the specular reflection on top of the diffuse color. It will be through the good understanding of how this works that you will be able to represent the true nature of a material (dielectric or conductor).
If you need to create a Conductor material it is simple. You just need to use the same color of the diffuse as the starting point for your specular map. So, if you are making something made of gold, your diffuse and specular will be of some shade of yellow, if it's copper, some sort of brown/orange etc. This way the base color of the object (diffuse) and the highlights (specular) will be of similar hues. Of course you may change the intensity and even vary the hue a bit to achieve the specific look you are trying to get. But you will probably want to stay close to the original diffuse hue range (Fig.14).
But in the case of a Dielectric material, the light that is reflected as specularity is not affected by the diffuse color of the material. It maintains its neutrality. If the light is white, the reflection will be white, if the light is blue, the reflection will be blue etc. So, in this case, it would be reasonable to assume that the specular map for dielectric materials should have no colors at all, right? It should be grayscale... correct?
Well, not really. In order to get a good non-metallic result (neutral specular color), we need to neutralize the specular reflection using the inverse color of the diffuse (Fig.15).
Why? Because in RGB, whites and grays contain all colors. Since they have all colors, they will also contain the diffuse color, and once you add this portion of the diffuse color, contained in the grays of the specular, on top of the original diffuse, the result will give the material a metallic aspect. As you can see here (Fig.16).
If you add a green specular on top of a green diffuse, the specular reflection will be a bright saturated green, which is a very metallic characteristic. If you add a gray specular on top of a green diffuse, the specular highlight will still be tinted green, not as much as before, but still making a metallic, conductor, aspect. But, if you add a specular color that neutralizes the diffuse color, such as this magenta, the end result is neutral, in this case, white. That means that the color of the light will not be affected when reflected as specular highlights, giving you the impression of a dielectric material. Compare Fig.17. See how it completely changes the nature of the materials?
Alright, so now we know how different types of materials affect light in specific ways. And we also know that, in 3D, the specular highlight is the result of the specular color being added to the diffuse color... But how can one benefit from this?
The great thing about it is that once you understand this you have a solid starting point, instead of just guessing what color you should use. And that saves a lot of time!
So, as a general rule for creating your specular maps the first step is to use your diffuse texture as a guide, and separate the areas that should be metallic looking and the ones that should be dielectrics.
With the dielectric selection, create a Hue and Saturation adjustment layer and simply slide the hue slider all the way to the end. This will shift the hue to the inverse hue value, the one that neutralizes the diffuse color under the specular reflection (Fig.18).
You will still need to adjust all the other values, such as brightness, contrast, levels, any specific detail you may need. And, of course, you may want to change the hues to achieve a specific aspect that you are looking for, but in terms of first step for the colors, this is a pretty good start.
But what if, besides all this, you could also get a preview of how the specular reflection will look like when rendered, directly in Photoshop, or any other software with layers and blending modes? That can save some time as well.
The cool thing is that, you can!
As mentioned before, in order to see the result of the specular reflection on top of the diffuse reflection you just need to add their values. You can do that in Photoshop using the Linear Dodge blending mode. This mode simply adds the values of the current layer and what's underneath it. So, if you paste your diffuse texture on top of all layers of your specular texture, and set it to Linear Dodge, you will see the color of the final specular render under a white light.