If you are not selling the queen, it really does not matter what colour you use. As long as you know what it means. When I started marking in 2018, the only colour I had was white. I knew they were 2018 queens. I bought a red Posca pen later that year. Now the really important question, in the northern hemisphere, we mark the queens on the back of their thorax. Since you are in the southern hemisphere, you marked her on the underside of her thorax, yes?