If you need 20 ounces, ratio the mixture with the same ratio. For example
3 ounces blue
2 ounces yellow = 5 ounces mixture
If you want a the same ratio with 20 ounces, multiply by 4 to get
12 ounces blue
8 ounces yellow = 20 ounces So, you would need 8 ounces of yellow or 6 more ounces of yellow!