changing colors by button problem

[Diddlybabble]

ay mates i wanted to change the screen color by pressing a button but it doesn't work anyways here is it code :color=0
function TIC()
cls(color)
if btnp(4) then
color = color+1
elseif btnp (13) then
color = color-1
end
end

[nesbox]

Tyr this

color=0
function TIC()
cls(color)
if btnp(2) then
color = color+1
elseif btnp (3) then
color = color-1
end
end