[Ak0]

I might as well give it a try

if someNumber <= 20 then -- If someNumber is less than or equal to 20
    STAddonLabelHTD:SetColor(0,1,0,1) -- Set the color to green
else -- If someNumber is anything else (e.g 21 and up)
    STAddonLabelHTD:SetColor(1,0,0,1) -- Set the color to red
end

Without checking I believe that should do it.