Crossword Genius

It made old people look blue (4)

Ross

I believe the answer is:

woad

I'm a little stuck... Click here to teach me more about this clue!

'blue' is the definition.
(thesaurus)

'it made old people look' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is present in the answer.
The remaining letters 'wad' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for woad that I've seen before include "Blue dyestuff" , "Plant yielding blue dye" , "Old British blue dye" , "Colouring" , "Ancient blue dye" .)

I've seen this clue in the Evening Standard.
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