Fixing shadow colors in MtoA ShadowMatte shader

[mayaEU6MH]

Hi there,

I'm trying to match an IBL skydome light and a backplate to create realistic shadows on a plane with the ShadowMatte shader. I've matched the exposure and color of my HDR IBL and backplate, and have configured a scene with some test spheres casting shadows. However, I'm seeing a very noticeable blue color tint in the shadows on the plane I've assigned the shadow matte to, and I can't figure out how to fix it.

I've established that the blue color tint is coming from the blue of the sky in the IBL, because if I shift the hue of the skybox image by 180° to make everything yellow, I see yellow-tinted shadows instead of blue-tinted shadows. That said, as far as I can tell, the IBL's coloring is "correct" - or at least, it matches the backplate.

Here's an 8-bit version of the backplate and IBL, for context. (The original images are EXRs, and the skybox IBL contains the full dynamic range of the scene, including an unclipped sun.)

As a test, I tried setting a different shadow color (other than black) in the skybox settings / in the shadow matte shader settings. In both cases, if I set a light shadow color, the color is used by Arnold without a blue tint, but the shadows don't look realistic (because the shadows in the scene aren't light). However, if I try setting a near-but-not-quite-black shadow color, sampled from the backplate image, it doesn't seem to get used, and the strong blue tint is just as visible as with a 100%-black shadow color.

What's the correct way to match the shadow matte shadows to the backplate shadows, without modifying the IBL's sky coloring? Or is there some other trick I'm missing?