This is my first tip, initially discussed here https://www.tt-forums.net/viewtopic.php?p=1172243#p1172243
for adding texture to the large surfaces.
In principle it consist applying noise layer to the uniformly colored surface.
I generated a block of noise (GIMP->Noise->HSV Noise, brown background, desaturated later, if you want to experiment, I had to tweak it a little to get the desired results, if I recall correctly, I mostly changed Hue value as others added undesired characteristics to the noise, but it still may be good for your use case).
File comment: Noise
Noise.png [ 124.59 KiB | Viewed 662 times ]
You can take this already generated noise sample or experiment with your own parameters.
Then put this noise layer above the surface you want to add texture and use appropriate selection tool to select only desired elements. Change the layer type to Burn layer.
Do not be disappointed when you do not see any effect or if your graphics turns just dark. You still have to tweak the result by changing opacity and brightness and contrast (for example with the curves tool) to get the desired result.
Here I applied this technique to my IKEA Arlon graphics.
File comment: Example 1
IKEA Arlon with noise.png [77.81 KiB]
Not downloaded yet
You are not limited by the roofs. Quast65 also experimented with adding it to the whole building here https://www.tt-forums.net/viewtopic.php?p=1172258#p1172258
with surprisingly good results.
Here is my attempt to generate some grass for the games field.
File comment: Example 2
Court.png [ 4.43 KiB | Viewed 662 times ]