This is one of those things that everybody knows, right? You’ve known since kindergarten. White sheet of paper. Yellow crayon. No problem.
I know it’s so because I’ve lost count of the number of times that I’ve read about “Earth’s yellow star.” It’s one of those things we take for granted or that we take on faith. A lot of things are like that today. Everybody says so, therefore it must be so. Especially if they stand up in front of a classroom, or if it’s been published in a book. Or they’re wearing a lab coat. Especially if they’re wearing a lab coat. Pffft!
The Sun is a white star. And yes, it’s the Sun. Just like the Earth is the Earth unless you’re speaking of the soil stuck to your boots, in which case you can go ahead and write “earth.” It’s capitalized if it’s a name just like it’s Mars, not mars. Or Moon, not moon, unless you mean a generic moon. Or a generic sun as in a “million suns.” Or Jupiter’s outer moons. But I digress.
Don’t take my word for it.
If your VPC (viewpoint character) has never seen the Sun except through dust and he thinks it’s orange, that’s fine, but if your VPC is a starship captain or a scientist or an omni narrator, he really ought to know better.