I keep getting the error "bad cast". what is causing it and how do i fix it?

I'm not too familiar to the new Lua works of Roblox and I couldn't find out anything about it on content help. this is the script segment that gives me the error:

local   trail = Instance.new("Trail", sun)
    trail.Color = ColorSequence.new(BrickColor.new("Royal purple").Color, BrickColor.new("Toothpaste").Color)
    trail.Attachment0 = sun

I just need to know how to fix it because this isn't the first time this has happened. every time I click the tool the error shows and I don't know how to fix it