Javascript Validation of two dropdowns for third dropdown?

Hello,

I need to have multiple dropdown selections populate a third dropdown based on the previous selections.  I have dropdown 1 as product type and dropdown 2 as finish.  Dropdown 3 will be color options based on the product and finish.  Considering there are different products with the same finish but different color options.  I was wondering which JS code I can use to check the product and finish.  For Example, Product A and Product B can both have a smooth finish, but where Product A can be produced in red, green, blue smooth finish, Product B is red, black, white smooth finish.  Until this point I'd have been able to use case "smooth" for the color options, but since smooth needs to have two different sets based on product, I can't.  I've been having trouble figuring out how exactly what code to use to check if product = A, AND finish = smooth, then color = red, green, blue.