I feel your pain. I have dark colored upper lip! I have always battled this problem since my teenage. Here is what I do for lightening my lips:
• Mix granulated sugar with olive oil to make a paste.
• Apply the paste on your lips and scrub in a circular motion and wash it off with luke-warm water. This process will remove all dead skin from lips.
• Apply beet juice on your lips every day. This will help in giving a pink color naturally.
• Apply lip balm after scrubbing and also before bed.

When applying lipstick; use some foundation/ concealer to cover your lips and then apply lip color. This will even out your lips and give an even tone.