If you would ask to make a scene complete black and white with the exception of one colour, it can be done very easily with the secondary color correction tool.
However, in your example, multiple colours are kept in the final result. The way you do it (masking by hand) is probably the best way to do and costs a lot of time.
Depending on the footage, other ways might be possible. I looked at the internet for a picture that resembles yours and tried to get your effect (keeping the colours in the flags only) without masking frame by frame. The red colour was easy, but removing the light blue sky while keeping the dark blue in the flags requires playing with settings for limit luminance and limit saturationin the secondary colour correction tool. But all together, this effect takes less then 1 minute.
First you see a snapshot of the original picture and then the snapshot of the picture after editing in Vegas.
Is that what you are looking for?
original:

edited in Vegas:

Theo