I actually used to add a little lemon juice to my buttercream in order to cut the sweetness. I never really tasted the lemon. I'm wondering if you would be better off using a lemon extract or a lemon oil for flavor. I'm just afraid you're going to have to add way too much lemon juice in order to get the flavor you want. Plus, depending on what color you are going to want eventually, the acid in the lemon juice can affect it. For example, if you are trying for purple, I think you end up getting red or pink. HTH
If you find that it's not "lemony" enough with just the lemon juice, I would suggest you add some oil or extract. My number one requested bc flavor in the summer is lemon and the only change I make to my normal bc recipe is to add lemon oil as the flavoring.
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