One of the more important issues with texturing for Urban Terror is dirtying textures up to make them more believable, more real. Sometimes you can start off with a photograph of something very clean, such as a wall in a corporate office - then mix that with an extremely dirty wall from the ghetto.
In fact, that is how I created most of my textures for Casa, with a source photograph I took of a nice clean texture - then dirtied it up with some great found photographs online. The first step is to take a mostly clean texture. For this step we will choose the white brick texture from the last tutorial. Next, you are going to want to find a grungy texture to mix with. I usually try several dirty textures, but for the purposes of this tutorial I will only use one.
Once again I raid Jeremy Englemans Public Textures and find this great texture, perfect for mixing with our brick.
The dirty texture we will use to fix up our boring brick.
Image cropped, tiled and ready for use as a new layer on top of our brick.
From here you need to experiment with the differend blend modes of the layer. I normally use the Overlay mode, then tweak the levels of the dirty layer to achieve the look I want.
Here I have selected overlay and noticed that the dirt does not show up very well, so I will fix that in the next step.
Set to overlay mode, not visable enough yet.
In order for this texture to show up more, I adjust the levels by hitting Ctrl L, (image>adjust>levels) then moving the black and white input levels closer towards the middle grey. This increases the contrast and saturation of the entire image to where we can now start to see the dirt some more.
Adjusting the levels of the image so the dirt shows up more.
The contrast is good here, but the saturation just isn't very real. It's a bit too colorfull, so hit Ctrl U (image>adjust>hue/saturation)
Here I desaturated the image by about 50%.
After 50% desaturation.
From here I start to play with the opacity levels of the dirty texture layer, bringing the texture back down to a more normal level. I ended up using about a 65% opacity on this, the final texture.
The key is to not overdo it, you want something realistically dirty and random, but not so dirty that it looks unreal. (Unless of course, you are mapping a city dump, there would be probably a few places that an extremely dirty texture would be called for).
The final texture.