Depending on how you are accessing the rig, there may be a couple of solutions that might help.
If you are using AMP to download the character, all you should have to do is right click on the dozer .ma file in AMP and hit "Get Dependencies". This will download all the textures associated with Dozer and link them to the rig when the rig is referenced or imported into a shot.
If you downloaded the rig from the AM website, he should download in a zip file which needs to be extracted, however it is usually best to try extracting the files in the location you want to store the rig as moving the file afterwards can cause the textures to become disconnected. To test if this was the case, you could redownload the rig, extract it in one location, and try opening the rig. If the textures are still missing then the issue may be related to something else.
If neither of these solutions work, then you can always relink the textures manually. To do this, go to "Window > Rendering Editors > Hypershade". Open the Textures tab and find the texture node for either the tongue or pupils. Select one of them, open the attribute editor, and click on the file icon which will open up a browse window. Now you can look for the texture file which was downloaded with the rest of the file, select it, and hit okay.
Repeat this process for the next texture node. Afterwards you may need to hit the refresh button, but the texture should be relinked and the rig should have color again when you hit the 6 key.