I would just like to say that coming from the Indian sikh perspective I find that this racist behaviour towards you is trully abhorrent.I have never actually had the luck to meet a sikh of a different ethinticity to myself before, so ,therefore, I cannot indentify with the way in which you feel. I would just like to say that the way in which you are treated is simply disgusting. I hope you find a little solace in the fact that I(and there are many others as well!) do not conform to hating you due to your colour, I would not wish to be treated in this way. To me hating someone due to the degree of melanin in thier skin is immature and ignorant.
I think you should not let this behavouir detract from your faith in sikhism in any way . There are always going to be people who let their bigotry get in the way of seeing you who you truly are and if they do, you can see that it is better off not knowing them. And finially do not let the few who have racist views stop you from interacting with the sikh community as you will find that there is acceptance out there-well here anyway!