What if you find a asset pack that fits a style you want but you need to slightly modify something, like modify green grass to be like a yellow sand color? is that allowed?

Yes, modifying works from OGA is absolutely allowed!  In fact, it's encouraged!   Open Art is meant to be modified, altered, experimented with, torn apart, re-worked, etc. etc.
If your changes are substantial, please consider submitting them back to OGA so others can use them as well.  But if you are just doing recolors, don't worry about that.  
In  either event, just be sure to give proper credit to the creator of the original work (see the rules for an example of how to properly credit a work).