shader for gui textures that render a precise color as transparent

Hello! We are developing a mobile game and we have just found that a pot texture without alpha channel does not boost the weight of out apk/ipa as one with alpha channel, and we we're wondering if someone knows or can write a "simple shader" that takes one color (magenta for example) and converts it to transparent.

Or maybe there are other methods that we ignore?

Thank you for your time.