Yes, the one on the right is a promo tile, which realy means it should be sold as a promotion of some sort due to inferior quality.
To determine the quality of travertine you are looking at the level of fillers used in the stone, this includes the colour too. When fillers are used they should realy be as close to the colour of the stone as possible in better grades. Transparent/ brownish types of filler dont look great in most travertine.
You should also look at the pits/ holes in the back of the tile. The less, the better. These holes get filled with you adhesive when installing the tile, your tiler will skim the back of all tiles to ensure that they are filled and fit them on to a bed of adhesive. It is reccomended to use white adhesive for this to help eliminate the chances of darker adhesives coming through the stone.
Although it is very true to say that shade will be different in natural products I would reccomend that you look out for excessive shade variation as this can be an indication of low grade tiles.
Hope this helps
hope you get it all sorted out