@DAVES
I had the same issue as you but I only have a HDS unit. I found that the more "pastel" shades gave the best background. I've set mine up so the colours change every 2 metres. On one map I have a range of 32 metres so I have a 16 colour palette, ranging from pink in the shallows to darker blue at the depths. This is contrary to what a lot of forum members have done but I find it easier to see.
I do a lot of trolling with lures so a quick glance at the plotter tells me if I'm heading for a shallower reef.
Not sure if this helps you but here are the palette numbers I'm currently using (92, 62, 98, 61, 97, 68, 16, 176, 11, 73, 182, 218, 253, 252, 18, 240).
Meggs