Found this on OpticsPlanet for the 9/40 Shield. At $225.00 it's not cheap. Suppose to ship in 8-19 days.
No info on the LaserMax site about this green laser.
Just a matter of time before Crimson Trace comes out with one for the Shield, maybe?
Lasermax CenterFire Green Laser FREE S&H CF-KAHR-PM9-G, CF-LCP-G, CF-LCR-G, CF-NANO-G, CF-SHIELD-G. Lasermax Green Laser Sights.

red for $85, green for $225... yikes. I assume there may be other differences, or is green always carry that much price premium?

From what I've seen, the greens are running about three times the cost of the reds. I have a red laser on my Shield for when it's too dark to use the fiber optic sights, also on my Shield.
Lasers can fail, sights won't.

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NRA R.S.O. & Life Member

Does anyone know if a Viridian green laser will fit my Shield 9?

Yes it will.
Here is the info from Viridian.
Viridian R5-Shield Gun Laser Sights | Viridian Green Laser Gun Sights
And here is the best price from opticsplanet for it at $152.99.
Viridian Green Lasers Reactor 5 Green Laser Sight for Smith & Wesson M&P Shield with ECR FREE S&H R5-Shield. Viridian Green Lasers Green Laser Sights.

I've got the LaserMax Genesis green laser on my .40c. Also have a BG .380 with the built-in red.
There is no comparison.
The red is almost useless outdoors unless its deep dusk or full dark.
Green is completely visible in full daylight for at least 50m, maybe more. In dusk or dark its so bright, you might not need to shoot because the shock value is extreme.
If money is not an issue, go green.