Testing your pack now to see if it does the overlay for me.
Could just be a hue thing with the particles?
I'll edit this when I have loaded in and tested.
--EDIT--
Tested it and it seems to be a vanilla thing, just for some reason it is more noticeable for your texture pack.
The colours of the damage hearts normally are much darker so might be harder to notice the shift in hue.
I am not sure of a fix, but at least it's not too horrible of an issue to make it unplayable, or is it???
Perhaps try changing the particle hue a little to see if you can find one you like. I just checked with another resource pack, and it does not do the same amount of colour shift, but it did shift a little. I would just play around with the texture till you have it to what you like. Look in other resource packs to see what they did with the particles also. I think you might want to fill in the particle colours, the example pack I compared to the damage heart was solid colour (Well shaded but still not white).
Hope that was helpful and you figure out how you want to fix it.
