It's a minefield, but David Griffen covers it on pages 86 and 87 of Ship Models from Kits. I work in 700 and 1200 scale, and for 700 scale lighten the paints with a light blue-grey and use the square root formula as a guide. What you have to watch are colours such as red, yellow, and orange for they don't take kindly to lightening with white or blue grey.
For 1200 scale, I use a colour that "looks right", and I'll use a specific "for instance" here. I'm finishing off a 1200 scale Titanic and instead of using black for the hull I am using Vallejo's Black-Grey, and for the white Vallejo's Off-White". They are not as intense as black or white and soften the appearance of the model.
In the end it is what you are comfortable with. I'm not a stickler for getting the colour exactly right and have found using exact matches can make a model look dull. I remember painting a 700 scale model in Measure 21 using correct matches; accurate but it looked awful - too dark. Some years ago, I painted a 1200 scale Hood in 507B, which was thought correct at the time, and it looked pretty dull too.