Joost, first of all, as to the colors per se, I like the ones I see in the empty tank, at least at the distance seen in the pics. A little more lighting may help us visualize things better and maybe a little closer... but I like the colors as you have them. If you feel you need an additional layer of clear epoxic resin, I would just brush it over the stonework as it is.
If you want the right color of water, the reddish-brown blackwater, all you need is Tetra Blackwater Extract or Kent Blackwater Expert, Catappa leaf extract and the right lighting, warm toned fluorescents, or you might look into the new red LCD lights now on the market:
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All you need to do is a little bit of chromatic play to get just the right deep red tea color you want... or throw a pipe from the Ventuari and get some real firewater.
Actually, once you get the right lights, you may not even need to add any tannins and trace elements will be all you need. Though a nice bunch of catappa leaves and the help of the two blackwater products I mention are always beneficial to the fish.