It is a 15-91 with the new design of the body. Some parts are modified slightly, but generally all it's features and strengths are the same. I can compare to the similarities between a black, cast iron, belt driven 201K and a beige aluminium 201K-23. I think some 15-125s are cast aluminium.
The paler colored versions can have some advantages when it comes to reflecting light, sewing black thread on black fabric, using a black machine can require an extra light source. I know some seek out paler colored machine for that reason, I guess most of us are happy with the new LED bulbs or an extra reading lamp when needed.
As a portable, the 201K-23 is much easier to move from one spot to another, it is still the same stable, smooth running work horse what ever the job you give it. The paler green color is attractive to some. Here these machines tend to go for about the equivalent of $30 to $80, but I seen very good condition machines with lots of accessories go for the equivalent of $170 or more. They often come with the Swiss zigzagger and it is still sought after.
Don't pay too much for a worn machine with out all parts there, they are well worth the fix up in general and touch up to the paint, but it requires more time and effort, and it should be relfected in the price. If you really want a green one, you probably find your self searching for years before you find one locally. In some areas they are more common than others, here I get the brown and tan verison a bit more often, but usually black. Machines with potted motors are the odd one out here.
There aren't much money in this, the worth is mostly for those who need a good sewing machine, the old domestic all metal straight stitchers are in a league of their own in many respects. Fixing up an old machine, is fun and rewarding and for things like free motion quilting and emboridery any 15 is a favorite.