Here is my Waltham Colonial 17j, dated 1914.

It has a yellow celluloid(?) crystal.

Could a yellow coloured crystal possibly be original to the watch, or has it just aged and changed colour over the years? Or could this be a replacement that has been fitted subsequently?

I want to preserve authenticity.

The crystal

These very beautiful thin "clamshell 2-hinge cases had 2 "achilles heels";
1. The Crystal was about "hunter grade", meaning that in some areas it was below 1 mm thick, and very fragile.
2. The stem structure has a very weak "1/2-section" supporting weld. Be careful to wind and set it only with the case closed.
The crystal was replaced with a plastic which is yellowing with age. Some people actually like the "yellow", maybe because it helps mask metal dial infirmities.
A clear galss crystal will make the watch look thinner, and more as it was originally intended.

I would recommend replacing that "yellow" crystal as soon as possible. These older crystals were affected to exposure to UV light and when yellow tend to attract moisture out of the air. This will lead to rusting of the hands and the upper part of the cannon pinion.
You can replace the crystal with either glass or plastic. The newer plastic crystals do not yellow with age or exposure to UV light and will not cause rusting.

Thank you.

I have changed the crystal to a bevelled edge glass one and the result is very satisfactory. The watch looks much more presentable now.