Don't know why, someone I showed the pictures to said this:
I beg to differ, I think the lens lacks colour saturation. It does not capture colours and contrast well as compared to stock Canon or Nikon. Also to consider is the build of the lens.
For these third party lens, if anyone is even interested, I had much better results with Tokina ATX series lenses as compared to Sigma.
Something to do with his screen calibration perhaps... your pictures quality is very nice at least to my eyes.