Patrice, this can be really tough… I was designing a lot of graphics for the web and I continually got comments that the colors were dark and cold. I had no clue what the were talking about. They looked great to me, however, I started using a preset on my mac (same 13″ that you have) and it seems to make people a lot more happy with the results. It took me a few days to get used to it, but now it looks natural.
It was the sRGB IEC61966-2.1. I also tried the Adobe RGB (1998) and had good results as well. If neither works for you, I would go the more safe route of purchasing a calibrator.
Hope this helps a little,
Jonathan