How do people who have multiple armies from the same faction handle vehicles and big machines/monsters?

Say you have a Thousand Sons army that uses 3 Forgefiends and you decide to start a Night Lords or Iron Warriors or World Eater army which can also use said Forgefiends and has a completely different color scheme.

What do you do? Do you try to paint them in shared colors somehow? Do you just get another 3 Forgefiends and paint them in the proper colors? Or fuck it Thousand Son Forgefiends in a World Eater army.

Infernal Master (110pts)
Sorcerer (95pts)
Sorcerer (75pts)
Tzaangor Shaman (55pts)

5x Rubric Marines (100pts)
5x Rubric Marines (100pts)
5x Rubric Marines (100pts)
5x Rubric Marines (100pts)

5x Scarab Occult Terminators (180pts)
6x Tzaangor Enlightened (80pts)

Thoughts about this 1000pts list?
Also is the only way for Thousand Sons to kill bigger things is a Mutilah Vortex Beast? Everything they have seems low on S but with lots of attacks and fishing for 6s to auto wound.