If you need 2 parts red, 3 parts cream, and 1 part blue for the certain you want, and you have 4 liters of red then multiply all the numbers by two
For red: 2 parts × 2 = 4 litres
For cream: 3 parts × 2 = 6 litres
For blue: 1 part x 2 = 2 litres
You would need 6 litres of cream, and 2 litres of blue for the 4 litres you already have