Which Color3 constructor should I use?
Color3.fromRGB takes values from 0 to 255 and matches what Studio's Properties window shows, so it's the easiest to read. Color3.fromHex is handy when you copy colors from design tools. Color3.new uses values from 0 to 1, and Color3.fromHSV is useful for rainbow effects, because you can animate the hue.
Example: rainbow part
local part = script.Parent
local hue = 0
while true do
hue = (hue + 0.005) % 1
part.Color = Color3.fromHSV(hue, 1, 1)
task.wait()
end