gistfile1.txt

struct PeripheralCommand {
  union {
    struct {
      uint8_t red;
      uint8_t green;
      uint8_t blue;
    };
    struct {
      uint8_t hue;
      uint8_t sat;
      uint8_t bri;
    };
  };
};

PeripheralCommand cmd;
cmd.hue = 0;
Serial.println(cmd.hue);
cmd.red = 1;
Serial.println(cmd.hue);

Output:
0
1