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Only 3!

It may look as if the two arms of the 'X' use different shades of pink, but in fact the whole X only uses a single colour.

Explanation

The difference is that one arm of the X consists of pink squares that replace white squares in the background. Thus these pink squares are surrounded by green squares.

The other arm of the X also consists of pink squares, of an identical colour, but in this case they replace green squares in the background, and are therefore surrounded by white squares.

Painters have long known that the way a colour looks in a painting is affected not only by the actual shade of the colour itself, but also by the colours that surround it.

If we zoom in to the central part of the image it should become more obvious that the 'X' only uses one colour.