Hi James,
I am not sure what Lowrance OS your unit uses, or what AT5 versions it is compatible with.
For maximum compatibity, I would start off by using colours only from the "Common" colour scheme, which are the lowest numbered colours in the AT5 colour set. If you are exporting a map directly from a project (not via a user map), then use the AT5 palette with white and 2 shades of blue:
AT5 exports from ReefMaster use AT5 version 10. If your maps don't work, you can try exporting the map again as a set of Shapefiles, and running these through the IMC yourself using a lower Atlas version (there are two older versions supported). If this still doesn't work, you could always try the older LCM format, but this lacks some of the features present in AT5, such as filled depth area polygons.
Hopefully Greg will chime in with more specific details on what your unit can support.
Matt