Some colours may be trial and error as to what looks ok and what is easy to read, some make the needles disappear.
The brighter LED tend to run warmer, but probably less than original bulbs anyway. Get the type that have the 4 sides also lit rather than just the face. Otherwise you'll have nothing but hotspots and a lot of these gauges ran off the back lighting of the housing to show the faces. No need for wiring harness, just swap them over. You'll have limited dimming abilities due to being an LED. You can reach these without removing the gauge cluster, just takes a bit of wiggling for access.