Yellow isnt hot enough.

Sometimes that's because of grease and shit that's burning less efficiently, and you just need to let it run and it'll fix itself.

It doesn't look like that though, so maybe carbon is built up?

So you can try to clean it, use a toothpick.or something to make sure there's nothing clogged.

There's a chance the fuel is just old, but I'd imagine you checked that first and probably even a new container, but worth mentioning in case you haven't.