Q:

Why is a dead man like a plumber's candle?

Here I read the following about a man who had just died:

He was the ghastly pale of a plumber’s candle.

What exactly is meant? As far as I can google, a plumber's candle is just a shorter and thicker one, perhaps containing more stearic acid (source). What has all that to do with colour?

A:

The ghastly pale of a plumber's candle is not an idiom in English. But it's not difficult to understand, either.
The melting point of lead is 621.5° F. A plumber's candle, used to melt lead and keep it molten while he worked would need to be thicker to avoid needing to adjust the lead pot as the candle burned down.

The picture you see here is of a leaded wick (outlawed for a few years now), which serves to keep the wick upright and to increase the efficiency of wicking, so that the flame was hotter and the melt pool was larger. (Most candles today have a flat-braided wick that curls down towards the candle and self-consumes. The upright ones often needed trimming with scissors.)
The hottest part of the flame is the white flame just above the orange flame (above the blue flame). This can get to 2000°F. So the container for lead would be placed near the middle of the flame to get maximum contact with that part of the flame.
Plumbers generally worked in somewhat darker spaces before the advent of acetylene torches/etc. Imagine the (thicker) candle with a pot spreading the flame out across the bottom: less light from the flame to radiate outward (the "light" being "hidden" by the pot.) In the dark, what you would mostly see is the glowing wax stick in the dark. Strearic acid (which was the main ingredient of tallow candles) is very white, not yellow as pictured here (this might be a beeswax candle.)
So, it's not hard to imaging a pale white plumber's candle glowing in the dark appearing ghostly, or, if it was a man's skin, ghastly. The flame would be rendered less visible by the pot, and candle just glowing, kind-of disembodied from the flame, would seem eerie, out of common experience.
It's a nice simile. I'm glad you brought it up.

