Should my elderflower cordial be dark brown?
Depends what you mean by lots and it'll be more effective if used as a pesticide, which is how tobacco has been traditionally used in the garden. Beware, many of the recipes on the interweb say this is an organic pesticide: it isn't as it'll kill beneficial insects as well as the nasties. It'll also harm plants from the Solanaceae aka nightshade family, such as tomatoes, aubergines, potatoes and Solanus. Therefore I don't really recommend it, so I'm not including a link to how you can make your own.
If you have lots of weeds to clear, then using the fag-end [aka cigarette butt - Ed] to start a fire would probably be more effective, though you'll probably annoy your neighbours at the same time. This also assumes fires aren't totally banned by your local council anyway and you haven't set fire to anything you'd like to keep in the process.
* = apart from how to make Rhubarb and Ginger jam, which has been my top post for 14 months now and is currently over 50% of my search engine hits.