My mother is a hairdresser and has been "coaching" me my whole life; I've also been dying my hair everything from bleache blonde, pink, purple, brown, black, red, etc for the last 20 years or so.
So, here's the low down...you won't get what you want with a temporary dye, especially if you're trying to go lighter. Generally, it won't lighten your hair much, if at all, unless you bleach it first...which I recommend you NEVER do yourself. It's amazing what colors you can end up with!
And unless you have experience, I wouldn't dye your own hair either...it's very easy to miss spots, overdevelop spots, get your roots a different color, or end up with a color you REALLY didn't want. If you are doing a very neutral color close to your own, it's probably okay.
Good luck! :) After all that, I'd say...try what you want. Like you said, it's only hair. I've learned plenty of lessons about hair and dye through trial and error, much to my mother's chagrin. Hee hee!