Identify thresholds with colours in contourf

Hi, my problem is:

I've got a square matrix with number from -1k to +50k, I would like to print these with contourf identifying two thresholds with colours. For example if the cell(i,j) is less than X, I would like to see red colour instead if the cell(i,j) is greater than X, I would like to see green colour, in the middle orange. How can I do this?