first - you have to store 3 colors for every player. so something like that will be needed:
-- server side!!
addEventHandler("onPlayerJoin", getRootElement(), function()
local col = { math.random(0,255), math.random(0,255), math.random(0,255) }
setElementData(source, "color", col)
end)
-- then on drawing function (client):
local col=getElementData(player, "color")
local r=col[1]
local g=col[2]
local b=col[3]
dxDrawText (getPlayerName(player), sx, sy - offset, sx, sy - offset, tocolor(r,g,b,textalpha), textscale*NAMETAG_TEXTSIZE,"default", "center", "bottom", false, false, false )