The lemon juice could turn your hair orange :x  Basically put, any bleaching/lightening agent follows this pattern.

Black hair lifts to -> Browns -> Reds -> Oranges -> Yellows -> Grey/White

The sun's rays alone will be much less significant.  I also noticed that a chlorinated pool kicks so much color out of my hair, but it also feels icky in the process.  I only go swimming in a pool once every two weeks.  Also, a clarifying shampoo could help, but those seem to have SLS or SLES.

Putting lemon juice in any hair is adding acid to thehair; thus, the bleaching effect. Add the sun's rays and you've a good combination to not only change the hair color, but dry it out. With the effect of ultra-violet, the two go rather hand-in-hand.

Personally, I would not recommend this technique for acquiring sun "highlights" as you'll fight the dry frizzies for a lot longer than the change in color.

Just my two cents.