change color or edit in your photo editor, save as a PNG file with the same name.
now go back to simpe, right click on picture and import

commit and save.
repeat to all txtr that you want altered in your object.
Recheck your package to make sure you didn't miss anything, or forget to commit/save.
go to tools at the top and fix integrity to play if safe.
and save again.

     Now your ready to change your textures.

this part of the tutorial just cover's importing and
exporting.
If you need more help, look around MTS2 there
are several good tutorial, on changing your package
to fit your object.
Note: The changes made to simpe is in preparation
for the expansion pack coming out, and is necessary.
But for now color options are not available.
Go to Texture/plugin view
This is for simple objects using the same name of the
original object for the models/parts, if you change the
parts names you can find more advance tutorials to
explain how to do this. We will add to this tutorial later
coving more advance changes.
Click on Texture Image/plugin veiw, at the bottom you
will see a box with several TXTR files in it, scroll the
box down. to the very bottom, high light the last file there.
Right click the image on the right and Export, do not
change the name minimize simpe..

As I  made this object for this tutorial, I didn't put much
time into the mesh
and no time in the texture, but as you can see all came
out as I intended

Update all size.