It is not possible to control the LEDs separately using the official EV3 firmware. You would have to modify the kernel drivers. You can only use the predefined patterns shown here.
ULONG LEDPATTERNDATA[NO_OF_LEDS + 1][LEDPATTERNS] =
{ // LED_BLACK LED_GREEN LED_RED LED_ORANGE LED_GREEN_FLASH LED_RED_FLASH LED_ORANGE_FLASH LED_GREEN_PULSE LED_RED_PULSE LED_ORANGE_PULSE
{ 0x00000000, 0x00000000, 0xFFFFFFFF, 0xFFFFFFFF, 0b00000000000000000000000000000000, 0b00000000000000000111110000011111, 0b00000000000000000111110000011111, 0b00000000000000000000000000000000, 0b00000000000000000000000001110111, 0b00000000000000000000000001110111 }, // RR
{ 0x00000000, 0xFFFFFFFF, 0x00000000, 0xFFFFFFFF, 0b00000000000000000111110000011111, 0b00000000000000000000000000000000, 0b00000000000000000111110000011111, 0b00000000000000000000000001110111, 0b00000000000000000000000000000000, 0b00000000000000000000000001110111 }, // RG
{ 0x00000000, 0x00000000, 0xFFFFFFFF, 0xFFFFFFFF, 0b00000000000000000000000000000000, 0b00000000000000000111110000011111, 0b00000000000000000111110000011111, 0b00000000000000000000000000000000, 0b00000000000000000000000001110111, 0b00000000000000000000000001110111 }, // LR
{ 0x00000000, 0xFFFFFFFF, 0x00000000, 0xFFFFFFFF, 0b00000000000000000111110000011111, 0b00000000000000000000000000000000, 0b00000000000000000111110000011111, 0b00000000000000000000000001110111, 0b00000000000000000000000000000000, 0b00000000000000000000000001110111 }, // LG
{ 0 }
};