B9 sports trim 2015 interior lighting?

Standard white lighting strips, the multi-colour was a cost option.

Yep...

Can multi colour be added at a later date or does it have to be factory installed?

White LED interior lighting was standard for the 2016 MY.

The extended LED Interior lighting pack (multi coloured) was available on the SE, Sport and S Line. Option Code QQ2 - it cost £325 on the SE and £100 on the Sport and S Line.

The extended interior light pack (multicoloured) was also part of the Light and Vision Pack (option Code WB7, 9SB). This included Audi Virtual Cockpit and Matrix LED headlights and dynamic front and rear indications. The Pack was only available on S-Line cars and cost £975

They will be out there but they may take some hunting down.

So weird. Where I'm from, the lighting strips weren't available at all, on my model at least.
We had 3 trims, Standard, Design, and Sport. Each available with the optional "S-Line" package, or/and an optional "Black Edition" package. These gave you the S-line bumpers, D-shape steering wheel, and black trims, and each available with a choice of either the 1.4TFSI or 2.0TFSI.

So my car, which is the higher specc'd "Design" trim, which was optioned with full sat nav, virtual cockpit and Matrix LED headlights, still didn't come with LED strips in the interior (nor simple things like grocery bag hooks, PDC, etc). Everything is a purchasable optional extra. Kind of stupid.