I have never heard of a Longhorn Marans. I think someone is pulling your leg. Ok let's look at the colors.
Top left hand pic. the light bird is a Wheaten. I can't tell from this pic how correct her coloring is It looks pretty good. The dark bird is a Black Copper Marans
Right hand pic at the top : Those are 2 Black Tailed Buff Marans. They may have an extra dose of autosomal red. It would have been easier to see in the chick down which may have had a darker than usual "hue" to parts of it
2nd row of pic: The bird with the white neck and dark body is a Red Shouldered Silver Duckwing Marans. Some will call it a Silver Duckwing but it isn't because it has an extra dose of autosomal red which is causing the dark red in his shoulder patch. ( I used to own one). If you use this RSSD as a sire , you will pollute your flock with autosomal red and make it very difficult to breed any other color true. Autosomal Red is a sticky gene and hard to remove from a flock once it is introduced. The RSSD is not a regular Marans color. It is a unlooked for and unwantd result of trying to breed Gold and Silver Salmon Marans. It is discarded by wise breeders because it will pollute with autosomal red. It has no use in a breeding program where one is breeding birds to the Standard. If you got the RSSD as a chick, it would have been very difficult t tell it wasn't going to be a Silver Duckwing. That only shows up after the feathers grow in and we can see that extra dark shoulder patch.
The 2 light birds in that pic aren't really any color. They look like a mix of Wheaten and some other color modifiers. Honestly, they look like a product of a Back-Tailed Buff Marans mixed with a White Marans. Again, this would have been difficult to see in the chick down and only for sure once the feathers were in. Looks to me like someone has been experimenting with color breeding and sold you the results. But they are still Marans and should lay those dark eggs for you and taste delicious.
I just wouldn't breed any of them except maybe the Black Copper to another Black Copper or the Back-Tailed Buffs to another Back-Tailed Buff. Or Wheaten to another Wheaten cock from outside your flock. Do not cross the Black Copper with the Back-Tailed Buffs. That will just give you a genetic color mess. Plus you will be crossing alleles which will make it doubly hard to winnow out anything useful. Esp. , do not cross the Wheaten with the Black Copper. This has already been done in the breed's history and caused no end of trouble trying to winnow the Wheaten back out of the Black Copper.
Former Director of Archives Marans of America Club
Very well put i think the dark cock will be a wheaten based on the neck color is to light. The Black Copper Marans should be a dark wine red.