A larger file size (up to 150kb) would help your black background look smother and would show more details. Also, I notice that some of the bottles have some stuff on the cap and that is sort of distracting. I do like the choice of colors that you made - fits the challenge nicely.
Your concept is good here. We are unindated with colors all around us in our products. It is the most important thing in advertising to use the color as well as the shape of their containers to 'brand' the product to be easily recognised by sight without having to look at any label. Beauty products and bath products use pastels for a very valid reasons. Pastels are feminine, soft, and luxurious. It appeals to the majority of the persons pushing the carts. I think the bottles need to be full for a product shot. What black surface are you using? Paper? I would have cleaned up the blacks to remove specks. I also use felt or velvet for black since paper reflects more light and shows up specks more. Sorry for the long post.