Do not assume that your girlfriend would not like your dark penis. I do not think that your girlfriend would get surprised by seeing the color of penis, as she must know that penis color is darker than other parts of body because of its structure and the ability of shrink.
You cannot make your penis size bigger because it is not in your hand to give the desire shape. If you know the art of lovemaking then you can satisfy her so rest assured. Overall, you are normal and have to do nothing to make her happy just show your affection, love, care and warmness in the relationship.