For stylised shader on vellum: apply colour ramp to grassfield based on blade height

[chluaid]

For stylisation purposes I'm trying to assign a ramp (using P.y) to this vellum grass but it's just coming out white. Flipping/adjusting the ramp has no effect. Anyone know a reason this wouldn't be working?

I can't just use curveu because short grass or bending grass should be darker. So in effect I'm trying to get like a depth pass from a specific height (rather than camera), like a P.y world space ramp. Perhaps there's an AOV for this?