First off, I just want to say this forum is amazing. I only just discovered it last night, and I've spent a few hours reading through posts here. The level of expertise here and the professionalism are both exceptional. On to my question:

Say you have unused pairs at a telephone jack, and need to splice a new cable onto those pairs to extend them to a new location. (Not best practice I know, but sometimes necessary.) If you are taking the green and brown pairs of the existing cable, do you splice them to the like colours of the new cable, or to the blue and orange pairs?