It boils down to a matter of personal preference. There are modellers who prefer to skip the primer step and go straight to color coats. Personally, I prefer to prime my builds for a few key reasons -
1) The primer shows the flaws I may have left behind during finishing, giving me an opportunity to fix them before moving to color coats.
2) Primer improves the adhesion of whatever paint I apply later.
3) Probably most relevant to your situation, primer gives a solid, neutral color to paint over. Because your helo is molded in OD green and you are applying a much lighter color, the primer will allow the hue of the sandy yellow to come out evenly without the green plastic peeking through the paint or shifting the tone of the yellow.