One way to create grass textures for airfields is to take an in-game screenie of a field near the location of your airfield straight down from about 2500-500 ft in the air. Open this screenie in your paint program and select an area devoid of any conspicuous characteristics, as this makes tiling problematic (it creates a repetitive pattern). Duplicate this layer of grass as many times as needed to cover the grass area of your airfield. Don' forget to rotate it occasionaly to avoid repetitiveness; the grass should look as natural as possible. When you are done, merge the layers so you end up with a single grass layer in your paint program. All the other layers (runways, peritrack, sand etc.) go on top of the grass. Export as .bmp, convert to .dds (you know this) and place in the designated game folder.
After this, you need to add the .dds to a .mos file: Import a stock .mos file in the mos editor from the Aircraft and Vehicle SDK and replace the necessary .dds files with the ones you created for your airfield. Export and place this .mos file in the required game folder too.
Now fire up CFS3, check the colour and texture of your grass and see if it needs adjusting. You should not adjust the mos file, only the .dds file. This means changing the very basis of your grass (which is the screenie: select a different part of the surrounding fields or a different altitude), or it's colour in your paint program (play with the colour balance, contrast, make it lighter/darker etc.). After redoing your grass, you have to convert it to a .dds and check it in-game again. Do this until you are satisfied with the result. Of course you have to repeat the whole process for the winter version of your airfield too...
With patience and effort, it is possible to create a grass airfield that is hardly distinguishable from the stock scenery. Of course one could argue if the colour of the grass should match the CFS3 scenery. I like airfields in CFS3 better when they blend in with the scenery. But apparently a special kind of grass was selected to be used on the old grass-strip RAF airfields and it probably stuck out when viewed from above. But as the war progressed, the number of airfields needing to be build was probably more important than the type of grass used. Besides, as most airfields were hardened by then and they no longer relied on grass runways, I don't think they gave a d*mn in the end. I have photo's of US bomber bases with knee-high yellow grass between the hardstands that most certainly was not sown but simply nature taking over after the airfield was built. The base personnel however did often cut the grass in large strips on either side of the runway (or should have done that according to regulations). These are shown on AM airfield plans and would probably have been lighter in colour when viewed from above. In the end, it is all a matter of preference and this is the way I like doing them. Take a look at 1944 RAF Stoke Orchard for an example.
Frosty