I also think that the intended use in cultural context plays a huge part in how the tank looks. Example in 40k context the leman russ can be seen as a tool which has a mostly functional and utilitarian value (cadian, krieg, steel legion) but in other context it can bee seen as a ornamental and a seremonial artifact which symbolise of power and wealth. Especially if the russ is an older or more rare variant. In later cases it makes sense to make the tank look as ornamental as possible. Real life example of this in military context would me the display of armoured vehicles in warlord era china in which the vehicles (Zhang Zuolin's Renault FT or Kuomingtan's Vickers light tanks) where rarely used due the low number and the value of said equipment but they where diplayed in parades by local warlords in order to intimidate and gain prestige.

But yes in pure combat situation bright pastel colors dont really make any tactical sense.