Hello beano378,
The setting fro grayscale converts color images to black and shades of gray to reduce color usage but, it still uses color ink. To use only the large black ink tank, the item would need to be a black and white document and you would be printing single sided pages on plain paper. Printing with the setting for duplex turned on, with the option for grayscale turned on or with the paper type set to something other than plain paper, will use color ink.
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