The screen is black and white. Matt was able to create an .LCM map for me and it looks good on the GPS screen. Matt indicated there is an issue with the V1.8 RM program for outputting .LCM maps and there will be a fix released soon. Also, below is what he replied about using the .AT5 format for the Lowrance handheld GPS.
The IMC is Navico's tool for generating Lowrance maps. ReefMaster runs it automatically but it is also possible to run it on its own, using Shapefiles that have been exported from ReefMaster. This gives a bit more flexibility, as you can set some options that are not available in RM, such as earlier AT5 (Atlas) versions. My guess is that version 7 might work on your system, and this would potentially give a better looking map than is possible with LCM.