Sift icing sugar into large bowl. Mix in milk and lemon juice until smooth. The icing should be the thickness of honey. If texture is too thick, stir in additional milk 1 tsp (5 mL) at a time. If icing is too thin, stir in additional sifted icing sugar, about 2 tbsp (30 mL) at a time.
Divide icing into 5 or 6 small dishes or cups. Mix a few drops food colouring into each dish to create assorted colours.
Use small, clean paintbrushes — one for each colour — to brush icing onto plain sugar cookies. Keep brushes in their own dish to avoid mixing up colours. To avoid smearing, try not to brush additional colours on top of wet icing. Icing on cookies will harden after about 15 min.