Because this is a photorealistic building, the textures have to be chosen carefully so that they do not contain any artefacts due to compression. The artist has to closely follow the colour provided in the concept art or, as is the case here, the reference photos if the concept art is not collared. This particular building is part of a desert environment, so the tone of the textures has to match the desert climate.
When choosing the base textures, the most important things to follow are the material details, as the colours can always be modified later. Another good idea would be to choose textures that have a lot of colour variation, as this colour variation will add a lot of personality to the building.
Here's how the wall texture for the basic shape of the building was made (Fig.06). The top image is the original texture while the lower one is the result after tweaking the colours with Colour Adjustment and a bit of Unsharp Mask.
The next image shows how I added different colour tones and a layer of paint to the existing wall (Fig.07). The easiest way is to select the necessary details with Colour Range and add them to our PSD. I use this method a lot.
Next up I used a mask (top frame) to add some details from a texture (middle frame) to our map. The result can be seen in the lower frame (Fig.08). The blending mode for that layer will be Normal. This will ensure that the newly created layer will not modify the original texture's colour.
Most of the details can be added in this manner by using masks or Colour Range to isolate desirable details from one texture and add them to our own. The final colour can always be modified using Replace Colour if needed.
One very important thing to take into consideration is the texture resolution. The artist must always be careful when scaling the source textures to keep both the natural scale of the details while at the same time ensuring that particular details will not be sharper or blurrier than the rest of the texture.
Another thing to consider is the management of the PSD files. This will help obtain the maximum texture quality but also decrease execution time when working on a tight schedule. All layers should be named and organised in groups to make sure all the details are easy to find in case they are needed. I try to keep the layers on Normal as much as possible to make sure I keep as much detail from the original texture while avoiding colour deviation and artefacts caused by using other blending modes like Hard Light or Overlay. If these blending modes are necessary, it is very important to keep the resulting texture as photorealistic as possible. Soft light seems to be a blending mode suitable for accentuating highlights, shadows, bevels, etc.
Here is one example that follows the above mentioned (Fig.09).
This next picture shows the final wall texture with a bit of saturation added to it (Fig.10).
The two Photoshop guidelines mark the lines that define how the texture is tiled. I needed to make the texture tileable on these lines as the wall is 5.5 metres tall, but I only had 4 metres' worth of texture to work with. The result is that I had to cut the geometry at 3 metres in height and tile the image at that height to increase the usable surface area. This technique is very useful when texturing a high building, but you also need to have dripping details at the top and dirt like details at and bottom of the texture. So basically, the middle part (between the guidelines) can be tiled to cover as much space on the geometry as needed, whilst keeping the top and bottom unique.
Here is the same texture applied to the geometry (Fig.11).