Thank you for helping me out of my brain derp. Brain wanted the last two params to be ending x and y point of the rect.
The following code now works:
background(255);
int fillColor = 0;
int gridDimensions = 32;
for (int i = 0; i < gridDimensions; i++) {
if (gridDimensions % 2 == 0)fillColor ^= 1;
for (int j = 0; j < gridDimensions; j++) {
fillColor ^= 1;
fill(fillColor*255, 50);
rect(
i*width/gridDimensions,
j*height/gridDimensions,
width/gridDimensions,
height/gridDimensions);
}
}