Shampoo
Shampoo does one thing: clean your hair and scalp. The difference lies in how aggressively that happens and what else is in it. Sulfate-free formulas cleanse more gently and are gentler on colored hair; a conditioning shampoo leaves a bit more behind. Wash the scalp, not the lengths — they get clean on their own when you rinse.
You choose based on your scalp, not your ends. If you get greasy quickly, you can wash more often and want a shampoo that cleanses well. If your scalp is dry or sensitive, choose something gentle and wash less often. You address the condition of your ends with a conditioner or mask, not shampoo.
Two cases where you need something different. If blonde or gray hair turns yellow, a silver shampoo helps — but not every wash, or it will turn into a cool haze. And if your hair feels limp while products no longer seem to work, there is usually silicone or styling product buildup; a deep-cleansing shampoo removes it in one go.
How often you wash is not a fixed number. Those who exercise daily wash daily, and that is fine with a gentle shampoo. Lathering twice is only necessary if you had a lot of product in your hair.