well when using color object you are setting its diffuse color which only shows when there is a light source
here is an improved version you could use
sync on
sync rate 100
// clears the screen to red
cls rgb(255,0,0)
// copys the specified section
get image 1,10,10,100,100
color backdrop rgb(0,0,0)
make object triangle 1,-10,-10,0,-10,10,0,10,10,0
// next part isnt really needed
` color object 1,rgb(255,0,0)
// this stops light from shading the triangle so that the red stays the same
set object light 1,0
//textures the object with the image taken before
texture object 1,1
move camera -20
repeat
xrotate object 1, object angle x(1)+1
sync
until escapekey()=1
end