We know that Holi is a festival of colour, on this day people apply powder colour (Gulaal) and hit water gun (Pichkaari) on each other.
It is not necessary that to celebrate Holi you need a colour. Its the feeling of liveliness in a community which creates the sense of festival celebration.
It is the emotions which creates the sense of liveliness in a festival. When you apply colour to other person you make a bond with that person and the colour enhances its spirit.
A night before the main Holi celebrations, people burn fire which symbolizes the burning of evil. And the whole community participates in it regardless of their religion or caste.