A little confusing. Flour can be made from two types of wheat. Hard Wheat is usally a dark brown and has a high proiein. The hardness allows the starch to absorb more water after milling. It feels like fine sand and pours out of the packet. Soft Wheat is usually white and low in protein. It fells like talcum powder after millng and falls in lumps out of the packet. In Europe they use hard wheats that are locally grown to make bread. The proteins are quite low so they make the flour stronger by making the flour whiter, taking more of the bran out.
To measure it they burn a sample of flour and weigh the ash that is left. So a T55 French flour is whiter than a T65 flour. The 55 denotes 0.55% ash left. The UK uses stronger wheats from the Commonwelath and Americas rather than making flour whiter. That is why French Bread stales quicker, they use a low protein wheat that can not absorb water to make it last longer.
So below is a rough guide. The colour of the flour does not match the strength of the flour. "oo" grade italian flour can be for pasta (high protein) or cakes (low protein)