Display Amber/Orange on LED Strip with no control module?
The resistor value you want will vary based on the amount of green you want, or the amber in general you want to see. It's safe enough that you can try a bunch of resistor to figure it out. Anything from 10 ohms to 500 ohms should be enough. You can put it on the negative side without issue.

Amber is, in rgb hex values 255, 191, 0. So green is roughly 75% of full brightness. Reducing it just a bit should help produce a nice orange color.

Once you found which value works best for you, measure the voltage across the resistor, and using ohms law, you can find the  wattage needed for the resistor. P = (V^2 ) / R or V * I. The more current your led strip uses the higher wattage resistor you'll need.