Hypochlorous acid can oxidise and degenerate melosomes in hair and skin. Melosomes are organelles that can contain melanin (which is responsible for pigment). Thus chlorinated pools can assist you in fading your hair color but by degenerating the melosomes they can also increase UV sensitivity. Of course there are quite a few hair products that provide UV protection to compensate for any loss of natural protection. So whilst chlorinated pools are not natural, swimming occasionally may assist in both lightening your skin and hair color. But if you intend on spending great lengths of time in the sun, use UV protective products for your hair and skin.
Additionally, the friction of the water also has some effect on lightening hair color, but that is primarily due to cuticle damage.
Lemon juice may help but it won't be a big difference, I doubt it would even be noticeable. What colors are you trying to go from? Why don't you just use eyebrow liner in the proper color and blend it in?
Just use makeup or go to an actual salon. Don't try yourself
You could try lemon juice, but it will not work wonders. It's a but unclear what you want - is your hair dyed lighter than your natural color and you feel that your eyebrows don't match (as in blond hair and black eyebrows) or are you worried that people will think your eyebrows are too dark for your natural hair?