sorry, i had assumed you'd used colour offset 240-244 for the button and it was swapping the palette to the greyer colours at offsets 235-239, what's actually going on is you've used colours that are fine, but neglected to alter the information in interfaces.rul
while this file may seem impenetrable, rest assured, it is.
all you need to do is make a ruleset that changes buttonReserveNone from 67 to 131* to reflect the section of the palette you're using.
what this does is define the "mid" point of your colour range, around which colours will be swapped (130 will be transposed with 132, 129 with 133, and so on)
interfaces:
- type: battlescape
elements:
- id: buttonReserveNone
color: 131
*this may not be the exact value you're looking for, but it should be in the right ballpark.