First...the heart-leaf I bought is a color that I don't see on Google images. It is the same color of a kiwi or a green apple. Is this color common??
My actual question...to provide a little bit of humidity...is it ok for these to be sitting in a tray of water on top of pebbles? The water will be below the pebbles and never touching the plant pot.
It can be light green. It will usually get darker in lower light. It may become more yellow if it does not get enough nitrogen. Give it some house plant food once a month. Usually it is 1/4 strength solution of a water soluble food like Miracle Grow. It likes humidity and evenly moist soil. As longs as the roots are not constantly in the water it should be ok. I would clean out the tray once in a while anyway so bugs and algae won't grow in it.