Here's a picture of my screen:
In-game, when you discover a Super Mushroom, you see this sliding red mushroom. For a 1UP Mushroom, this experience is exactly the same, except the mushroom is green. However, while creating a level, the image representation of a 1UP mushroom has a "1UP" displayed on it. Why is that? The mushrooms are already differentiated by the colors red and green and the 1UP Mushroom in the game doesn't have this label. I have a theory.
When you play a Mario game and discover a mushroom, you almost always are going to grab it. Grabbing one of these mushrooms is almost always a good thing to do. You don't discern, you go for it, whether it's a Super Mushroom or a 1UP Mushroom. It at least doesn't hurt, even if you already are Super Mario (larger Mario) or have 99 lives.
Conversely, when you design a Mario level, you're going to care about your mushroom types.
Let's say you want to place a power-up in a particular place in a Mario level you're making. You decide you want the Super Mushroom. So, then you look through your arranged design elements, locate that red Super Mushroom, and place it into the level. No biggie.
Now let's say you're red-green color blind and then want to make sure to put a 1UP Mushroom and NOT a Super Mushroom into some secret part of the level. You might discover this issue: both the Super Mushroom and the 1UP Mushroom are only differentiated by the colors red and green.
That's where the "1UP" label comes in. Because of the existence of that label, red-green colorblind fans/players will not have a problem at all choosing the appropriate mushroom.
When you're designing Mario levels, you care about your mushroom types. When you're playing a Mario level, you don't really care as much. They're all "good things" to grab. So that's why that decision to put "1UP" as a label on a green mushroom in the level editor is necessary. This is a super-smart accessible design decision.
Good job, Nintendo. This makes me happy and proud.