Ok, so this can of course be done any number of ways, but it is a pretty complex task. I do some of this stuff for the AI already. Namely prioritizing cover, line of sight, accuracy etc. You could modify the same code and have your player units utilize it.
Essentially when you choose a target location you would sample tiles in an area around the target and use these as a filter for the AI functions, only looking for and weighting tiles within that area. That should also make this quite a bit more performant than the default AI, which considers all possible tiles in move range. You can find this code in the AI section of BP_Ability.
Regarding displaying tiles in move range with varying colors depending on the units that can reach it, you could spawn overlapping translucent meshes or decals, which would add together to give darker colors for tiles that multiple units can reach. For more control you could check the keys of each pathfinding output and see how many are able to reach each tile before spawning the appropriate marker.
Though if you are a professional in behavioral AI I’m sure you can think up better implementations than mine. If blueprints are your big hindrance here I can provide you with C++ versions of the pathfinding and visibility functions if you’re interested.
What are you trying to achieve with this particular funtion and is it working like you want it to? If not, what behavior are you seeeing instead?