food colouring vs food coloring

food colouring and food coloring both are nouns.

Word	Noun	Adjective	Verb	Adverb
food colouring	Yes	No	No	No
food coloring	Yes	No	No	No

As nouns, food colouring and food coloring are synonyms defined as:

food colouring and food coloring: a digestible substance used to give color to food

Other synonyms of food colouring include coloring, colouring, food color, food colour.

food colouring (noun)	food coloring (noun)
a digestible substance used to give color to food	a digestible substance used to give color to food