You can go to your "C:\Program Files\BobsTrackBuilder\XPacks" folder.
Unzip the default xpack.
Them go to the "Default\Textures\Ripple" folder & open the "ripple001.dds" file in photoshop or similar.
Modify it to suit your needs.
If you save as same name the sobject will just pick up your new texture.
Go back to the "default" folder (the one with CrossSections, Materials etc..)
Select all of these folders & re-zip.
Rename it to something other than default (maybe pezter).
Copy it back into the xpack folder & delete the "default" folder.
Now you should have a new xpack called "pezter" in your list.
The thumbnail of the sobject won't be updated but the one you see in BTB should be fine.
If you don't have photoshop try this great free program which has support for dds files inbuilt.
http://www.paint.net/
Cheers