Err..light brown hair will only look natural if your colouring suits it. If you do your entire head light brown, it won't look natural...(as light brown hair will have different tones etc) when the roots come through it won't look natural...if you have dark eyebrows it won't look natural.
Unless you are Madonna and have the right skin tone/money to afford top ups and an amazing colourist, it won't look natural. Sorry.
I've been to a really good salon on a whim in LKF..but you have to pay a packet. I actually (sadly) don't know the name of where I went to...(after a long period of not highlighting my hair) but it had a VIP room, an a colour technician called Bowie who did the work- her job was solely to apply dye. She did an excellent job. Sorry, I'm not of much use :-(.