Is it ‘coloured pencil’ or ‘colour pencil’?
You can say either. Both have entries in the Oxford English Dictionary, and of the two coloured pencil is the earlier, with a current first citation of 1735 (“This gum-water must be kept in a bottle always stopped close, and never dip a coloured pencil into it”), some 64 years before the current first example for colour pencil (“A few sets of the beautiful crayon colour pencils”).
The Oxford English Corpus, which monitors language as it is used in everyday contexts, shows that ‘coloured’ (or its US spelling ‘colored’) is much more common. There are parts of English-speaking Asia, however, in which ‘colour’ is the more dominant form.
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