I certainly used to use sets with only 2 colours of lamp.
Useually the very cheap imported multi-coloured sets of 20 lamps sold for as little as ?1 a set.
It was a simple matter to swap the lamps around to get any desired mixture.
Yellow and green was a favourite around the hallway, yellow gives enough light for safety on the stairs etc, as the Christmas lights were used instead of normal lighting, not as well.
Blue and white was attractive IMHO but not readily achieved with standard light sets since the greater brightness of the white lamps rendered the blue unnoticable.
I used to use 20 volt 3 watt blue lamps in series with 3.5 volt 0.15 amp torch bulbs, either 3 of each from 70 volts, or 10 of each from the mains.
The blue and white lamps were a bit more balanced in light output.
Another option for a large Christmas tree was seperate sets of blue and white lamps.
Sets of 12 lamps each 3 watts for blue, and sets of 40 lamps, each 0.75 watts for white.
The higher wattage of the blue lamps, and useing twice as many blue as white, give a good balance.
I remember the only aggravation being persauding a supplier that I wanted 144 lamps each 20 volt 3 watt ALL IN BLUE
