You are talking about two different things - but neither of them is a yam. Yams are rarely grown or sold in the United States, though you can often find them in Latin American markets. (There are more than 150 varieties of yam grown in the tropics. Some of them are sweet and orange, others are pink, purple, white, or yellow, and they can be dry and mealy when cooked).

What you are dealing with are two different species of sweet potato - the darker of which are often (mistakenly) called yams (and especially in the south). The light or “white” sweet potatoes you see are more potato-like than sweet-potato-like. They are dry, crumbly, and not particularly sweet after cooking. They are not really what you are looking for to make a sweet-potato casserole. The dark sweet potatoes are moister and sweeter when cooked, and have the expected orange color.

Finally, don’t believe everything you see on TV, especially when it comes to color. Lighting, photography, and styling all conspire to make some things look better and some things look worse than they do in real life.