I'm currently addressing the issue with HTC on chat, they had this to say -
Android 4.3 update - Thermal/dark noise and Pink/Purple camera hue issue] When taking pictures in low light conditions, you may find the thermal / dark noise and bright pattern on pictures. The symptom will be more obvious when the device temperature is high. Action: We have delivered the Android 4.3 update to improve the noise symptom and provide several enhancements. Follow the Android 4.3 update script to reply with users. (ETA 10/2013) If android 4.3 update has already rolled out,please upgrade the phone to the latest software version. Please note that it's an improvement, but not a total fix. If it does not help- Please perform a master reset. Factory reset is not a step to be taken lightly. It’ll permanently delete all your data and customized settings, and will remove any applications you’ve downloaded and installed Press and hold the Power and Volume down keys for approximately 15 seconds. Note: Performing a factory reset via hardware will automatically wipe ALL DATA from the phone, including the built in 32/64GB memory.Please back up data. Step 2: The screen will turn off, and then after a few seconds, the bootloader will appear, at which point release the Power and Volume down keys. Click Next to continue. Step 3: Press the Volume down key twice to highlight FACTORY RESET. Step 4: Press the Power key. Step 5: The phone is restored to factory settings.