iPhone 6 Plus Touch ID Sensor developing oily-like "rainbow colors" on its surface
There is a layer between the sensor and the glass that covers it, sometimes liquids can get in depending on pressure. It looks like an oily substance has seeped in between the sensor and the glass and no cleaning agent will therefore work (I was going to suggests isopropyl alcohol).

Try your warranty first, it's a cheap replacement at your local shaman otherwise.