…None of them have the combination of Red/Brown Mana colors.
If you’re going to continue to introduce new game gimmicks that restrict players options when participating in these events. Please rethink/rebalance the way you design troops.
This is just step 1 on necessary balance changes as the game continues to “evolve”.
all of the Red/Brown troops went to Bounty instead lol
of the 27 Bounty troops (out of 34?):
5 Red/Brown
4 Yellow/Blue
3 Yellow/Green
3 Yellow/Brown
2 Blue/Green
2 Blue/Brown
2 Green/Brown
1 Red/Purple
1 Red/Yellow
1 Red/Blue
1 Yellow/Purple
1 Green/Purple
1 Purple/Brown
0 Red/Green
0 Blue/Purple
Spoilers:
War Elephant is Green/Brown.
Gems of War has great troop color distributions. Just look at the 2 color converters 
There is also no Red/Green empowered troop. In addition, only 1 troop (Gimlet) is an empowered Brown converter. Other colors including skulls have at least 2 converters with Blue and Red having 3.
Although in the past, I have commented how I am not a fan of influx of empowered converters, they are an integral part of the game now. The devs just need to flesh things out a bit regarding troop colors as well as mana conversion colors. More Spirit Fox style troops might also be called for that deny the other 5 mana colors other than Yellow.
There is also Star Gazer for blue, but your larger point is still correct.