How do I make a block that changes colors loop those colors?

You can set a variable of the color, break the loop, then set the block's Color into the color we've just set on the variable

Make sure the script is inside the block

while true do
    script.Parent.Color = Color3.fromRGB(255, 0, 4) --change this to what color you want
    wait(0.5) -- change this to how long you want until it changes to the next color
    script.Parent.Color = Color3.fromRGB(255, 250, 87) --change this to what color you want
    wait(0.5) -- change this to how long you want until it go back to the first color
end

to add more color/s, paste this between the last wait() and the end() part

script.Parent.Color = Color3.fromRGB(125,231,17) --again change to what color you want
wait(0.5)

if you want the change of color to be smooth, try TweenService