Take my advice and pick up testor's Model Master modeler's technical guide. Where as you know most of the modeling technics they have an extensive cross reference section of all the popular brands colors in enamel and acrylics. They also sort them by era and country ie: wwII panzer colors.
For Panzer Grey they list it as color name RAL 7021 Panzer Schwarzgrau for 1939 to 1943.
Model masters ACR as 50123
tamiya as XF63
Gunze-San as H401
I hope this helps and pick up the book. They even have color swatches in the back.