What a stressful situation to find yourself! Tradition dictates that it is indeed the mother of the bride who chooses her outfit first and it is the grooms mothers job to then choose something to compliment this. It is not unusual for both mothers to wear similar colours but usually when the mother of the bride is OK with this! It is a difficult situation, but if your fiances mother is very unhappy with this prospect, then it really is for your mother to choose an alternative.

Traditionally the brides Mother would choose first and inform the grooms Mother in order for them not to clash. This also makes it easier on the day for guests to distinguish one from the other. If they are both insistent on wearing the same colour then the only other option would be for them to have different colour accessories.