The best chance would be to have a virus that targets specific [allel][1] that are found primarily in the ethnic group you are targeting.  

This wouldn't be fool proof, since many other *might* have it as well.  It would take some research to find such a sequence, and then on top of that, you need to design a virus that NEEDS that particular gene sequence in order to multiply. 

The 2nd part would be much more difficult, and with viruses no guarantees it wouldn't mutate.

Now, the one you suggest about chemicals being present, might work both as present or absent, say melanin.  It could be attracted to it to kill off those with darker skin or repulsed by it to kill of those with lighter skin.  And according to [this][2], there are 6 genes that affect melanin so picking one or two of those might narrow down a target ethnic group.  

EDT:
Didn't see the hard science tag, I am unaware of having that kind of manipulation and design ability in viruses at this time.  These are things that are theoretically possible.


  [1]: https://en.wikipedia.org/wiki/Allele
  [2]: http://anthro.palomar.edu/adapt/adapt_4.htm