This lesson could take place in the Winter - especially if it has snowed. If not, make a collection of photographs of winter landscapes. Also look at examples of work by an appropriate artist. Children in this project talked about winter landscapes by the Welsh painter Kyffin Williams.
Talk about snow. Ask the children what is the colour of snow. They will reply that it is white - then encourage them to look at the photographs and paintings to see if snow is always pure white.