This explained in the first set of 'hints and tips' for the course and through reminders in some others.
Welsh uses a verb-noun form for the base form of its verbs - similar to the -ing form in English, or a Latin gerund, but with a slightly different range of uses.
On the course we cannot do what a dictionary does for standalone words and indicate whether they are nouns, verbs, etc. To avoid the confusion we use the convention of translating standalone verb-nouns as 'xxxing' or 'to xxx'. So:
- lliw - colour, a colour
- lliwio - to colour, colouring