All options can be right depending on context. Without knowing context, I would expect "number seven is yellow" to be my most likely choice, however, if I'm misunderstood, then I will add "the" for emphasis.

1. “The number seven is yellow.” This is the most natural when talking about the number as an idea or symbol—like in synesthesia. Tip: Use “the” when talking about something general or known. 2. “A number seven is yellow.” This sounds like you’re describing one example—like seeing a yellow number seven painted somewhere. Tip: Use “a” for one example of something. 3. “Number seven is yellow.” Used when “number seven” is a label, like a player or item in a list. Tip: No article when using it as a title or label.

They all could be right because they are different descriptions. The first statement describes a number seven that is yellow, but there is only one to describe. The second is similar, but there are other sevens that are other colors such as on a game board. The third indicates the seventh item is yellow.