If you have a Unity project in versions up to 5.6.2 and on loading it into Unity 2017.1 or later some of the terrain textures turn reddish…
Take a look for an object in the scene or click on the terrain and select the terrain “Paint Texture” paint brush icon. Then select the texture that is rendered with the redish colour and select the image… I had one for example in
Assets > Terrain Demo Assets > Textures > Cliff (Layered Rock)
It showed as a bluish image rather than being a natural terrain colour. Find the problematic image in your project assets. In my case it showed as a bluish “512×512 DXTnm”, whereas the working terrain textures were “256×256 Compressed DXT”.
Search on Google, for example, to find a replacement for any broken terrain texture. One example location where Unity compatible terrain assets can be found is: http://github.com/leapmotion/unity
I imported the fixed image to the Unity project (actually a JPG version of it to save space) and then replaced the image in the materials that were reddish on mesh objects in the scene. For any terrain area that is reddish, click on the terrain and select the terrain “Paint Texture” paint brush icon… then select the faulty texture and replace its texture image.