You're right. Even weirder, this seems to happen only when you use colors in the red-yellow spectrum. It's definitely not cecho, though. It's something in Mudlet itself. The same thing happens when you do this:
local r, g, b = unpack(color_table.green)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.lawn_green)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.green_yellow)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.yellow)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.gold)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.orange)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.orange_red)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.red)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.violet_red)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.purple)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.dark_violet)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.violet_red)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.purple)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.dark_violet)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.violet_red)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()
r, g, b = unpack(color_table.purple)
setFgColor(0, 0, 0)
setBgColor(r, g, b)
echo" "
resetFormat()