Yep, there is no "supposed to". Some horses look better in a certain color but that does not mean that they HAVE to have that color. My saddle is a darkish brown and I use it on every color horse from gray to blood bay to dark bay to sorrel to black and they all look fine.
If you have the $$ to buy a saddle for every horse you have - go for it. Otherwise, buy a saddle that is comforatble for both you and your horse and don't worry about it. Of course, if your backside is as big as mine, the saddle color won't matter - 'cause no one will be able to see it!!! ;)
I have never heard of that I have a tan saddle I use on my chestnut appy, my black appy, and my buckskin paint and if they were a color scheme for them it would be a hard one to match cause they are realllly colorful just find one you like, and is comfortable to you and yor horse
You want your horse to be complimented with the colors you put on them , tan doesn't even look good on grey ......... whatever looks best but I think that grey horses look good in black , when I want to find a color for a specific colored horse I do like red roan horse being ridden in like google and then look at the pics and see what people put on there red roan and it is so easy that way so yeah I looked and there is just black saddles and like one brown saddle but he is like white .......