The actual color of the button will indicate if a swap can be initiated, a swap can be made as a counter offer, 

or if you have actually received a counter offer back for a swap that you initially created in the first instance.

a shift can be swapped.

this shift has been offered for a swap.

 Another shift is made available to swap with.

shift has been offered up for a swap. And other shifts are available.

 The swap has been agreed and is pending for approval from the planner/Traffic Manager

When the swap has been approved by the planner the button turns grey again, where it is possible to swap the shift again.