>What is the relationship between the first pair and the second ? or in the above example, what is a correct second value for the second pair ?

The goal here is to obtain the same degree of difference between both pairs. I do not have the correct value of the second pair. The first pair is a shade of green. The second pair is a shade of blue. From the first blue color, I would like to obtain the exact same degree of difference by calculating the second value. This is to offer themes to a Web site. By default, I would like to propose a set of colors when the main color is being changed. Then, this should do. However, the user still has the chance of adjusting the rest manually if needed. But, at first, this should do for most of the themes we are creating.