You can use NEUT+EXOT to make it grey, but then it can't be encased in anything but WALLS/DMND.
You can check the source code on github for that. EXOT is grayscale naturaly with life above 1000.
you can set the graphics function of EXOT with a Lua script:
function graphicsEXOT(i, colr, colg, colb)
do stuff here
end
tpt.graphics_func(graphicsEXOT, elements.DEFAULT_PT_EXOT)
Here is an example for "do stuff here":
f = sim.partProperty(i, "tmp")
return 0, ren.PMODE_FLAT + ren.PMODE_BLUR, 255, f, f, f, 255, 255, 255, 255
Theres 2 ways to make gray EXOT
1.-EXOT+NEUT
2.-EXOT+PROP that sets the life to 1000 or more