Found this today on the internets. I know there may not be a color standard, but this will do for now.
When documenting and such, I will refer to these colors.
General-Purpose Wire Colors onboard balloon payload
| Wire Function | Color |
|---|---|
| GND | Black |
| 5V | Red |
| 3.3V | Orange |
| SDA | Green |
| SCL | Brown |
| GPIO | Gray |
| SENSE (Ground Support) | Blue |
| TX (from Computer!) | Purple |
| RX (to Computer!) | Yellow |
You can find the source here