Philosophical intuition: just what is ‘a priori’ justification?
Philosophers use the term ‘intuition’ in a slightly different sense than it is used in everyday discourse. Generally speaking, the difference is that philosophical intuitions are based solely on understanding a proposition, while non-philosophical intuitions are not. If a proposition seems true to you simply on the basis of your understanding of it, and not on the basis of empirical evidence, testimony, memory or reasoning, then you are having an intuition in a philosophical sense that it is true.
‘Red is a colour’ seems true to you solely on the basis of your understanding that proposition. That seeming true is a philosophical intuition, and it is what justifies you in believing that red is a colour. Many, but not all, philosophers hold that a priori justification is based on philosophical intuitions.