ND Filter colour cast

Thanks for the suggestions of methods for dealing with this.

In the meantime I've had time to do some testing.

This is a "white" wall with that filter and AWB (or other presets). The result is basically the same with different exposure times (and appropriate aperture adjustments)

Setting a custom white balance produces a suitably grey result about 1 stop underexposed.

Unfortunately I didn't have a WB card travelling with me (although I perhaps could have used the white water???)

Would you say this is a faulty filter or something else?

I have limited experience with Curves and adjustment layers. Can you recommend a tutorial site?

Thanks again for the help.

N.

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nicholas