Clearly, wherever, blue is the most and yellow is cleary less.
That leads to the length of time at the gym.
In other words, yellow is clearly disadvantageous in almost every area.
And Niantic does not try to improve it.
Maybe the problem is you have a poor porcentage of yellow players in your area. Here we are quite equal, and sometimes I have the feeling that the players have agreed in leave every colour for 12 hours, so they can earn the 50 coins, and after there is a switch in all gym’s to change colours…