Colorful waistcoats with morning dress

I'm sorry I don't have sufficient specific knowledge to answer your question in detail, but I recall elaborate, fancy waistcoats originally having been one of the hallmarks of formal dress, and as time passed what was once dress of a lesser degree of formality succeeded to the role of higher levels, so I would expect that at different periods, colorful waistcoats have gone in and out of vogue with specific degrees of formality.

 

I couldn't tell you exactly when it started, however, by the time morning dress as we know it and its place in the hierarchy of formality in menswear solidified as it now sits in the 30s, it would not have been uncommon to see these pastel shades on display. Clearly less formal than black (most formal) or dove grey (next most so), and often to be seen with the less formal "morning suit" (i.e. trousers and morning coat both in grey).

 

The Georgians and Victorians (replace as appropriate for the 18th and 19th centuries in the US) often used bright colours in menswear - at least for those who could afford it. Edwardian fashions from 1901ish often seemed to become much more sober, at least for formalwear, with colour making a comeback into the late 20s/ 30s. Based on what I've seen anyhow. Be interested to read more on this; we do have daywear expertise lurking on the Lounge.

 

I don't know about day wear expertise, but I do have a rather talented missus. Edward mentioned that black was the most formal and grey the second. For what it's worth I think that a silver/white mix is both formal yet flamboyant, although you might be hard pushed to buy a waistcoat in such a colour. Like I said, a talented wife helps.