Need help with a checkerboard algorythm

[giladal]

Your parity is off. If you change the conditional to (x + y + 1)%2, it works. Or you could change your tests to be the reverse. That should work as well.

Wow, thank you very much for your help. It did indeed solve my issue.

Does that mean that changing the color to 255 and leaving it (x+y)%2 the color on the else statement to 255 will also help?

for y in range(n):
           if (x + y)%2:
               mat[x,y] = 255
           else:
               mat[x,y] = 0