RGB leds have three color DIODES: RED, GREEN and BLUE. It's the mixing of these three colors together that create other colors.
The color ORANGE is created by mixing RED + YELLOW.
The color YELLOW is created by mixing RED + GREEN.
With an RGB LED, there is only one RED diode so it can only be used once.
So what's the solution? Well, to get something that looks more like orange to the human eye using RGB LEDs you need to mix equal amounts of RED + GREEN (actually just a bit less green since the human eye is more sensitive to green wavelengths). This will give you the color YELLOW. Then, increase the intensity of the RED just a little (or, pull back the green). This will essentially add a little red to the yellow to effectively mix a color that has an orange tint. It's not perfect but it's as close as you can get to the color orange using RGB led lights.