First, you will need to create variants for your products. So in this case, you will have a t-shirt with the “colour” attribute, that will have two values: white and blue. You will also set the sales price to the price of the cheapest variant (in this case, the white one). 

The second step is to create a pricelist:

• Activate the pricelist & choose the Advanced price rules

• Price computation = Formula

• Based on = Sales Price

• Discount = -20% (because it is 1.2x more expensive)

• Apply on = Product Variant

• Product variant = The blue t-shirt

• This pricelist needs to be the default pricelist for a smoother flow

Now, when you will select your product on the quotation (if the correct pricelist is set on the quotation), you will see that the white t-shirt and the blue t-shirt do not have the same price, and that the blue t-shirt is indeed 1.2x more expensive than the white one. 

Note that you can do this for all of your variants. For example, in this case, we also have a rainbow tinted t-shirt, which is more complicated to tint than a regular blue one, so it actually is 1.6x more expensive than the white one. We only have to add a line to the pricelist for each variant that is more expensive than the regular one.