The best bread depends on your personal taste. Whole wheat bread and enriched white bread have similar nutritional analyses. Bread is a good source of thiamin, riboflavin, and niacin. However, whole wheat bread does provide significantly more fiber and iron.
Read the bread label. I recommend a whole wheat bread with whole-wheat flour as the first ingredient and no white flour added. You have to carefully read the label as wheat flour may be listed first. It is flour made from wheat without the bran coating. Next, the label should list only the other ingredients in bread such as water, sugar, shortening, salt, and yeast.
As to the color of bread, you can turn bread a dark brown by adding molasses which increases the iron content (not significant per slice though). The color does nothing to add fiber to your diet and I do encourage patients to increase the fiber in their diet by eating whole wheat bread instead of white bread.
White flour is nutritionally similar to whole-wheat flour. Whole-wheat flour is slightly higher in calcium, phosphorus, and potassium, but white flour is slightly higher in riboflavin. It is a food myth that white flour causes hyperactivity in children or that white flour is inferior to whole-wheat flour. The main difference between the two is fiber and whole-wheat flour is much higher in fiber.
