I have a set of colored plaques (flat square pieces of plastic) which range from opaque to transparent, and many are translucent: you can see shadows of objects held behind them if you hold them up to the light. My question concerns the terms translucent and transparent. If a plaque is less opaque, we say it is translucent. If its opacity is near zero, will it eventually be transparent? Is this true if the plaque is colored, say red, so that all red light passes through it, but not other colors?
Usage of the word "translucent" is inconsistent. Historically, according to the Oxford English Dictionary, until about the mid 18th century it simply meant transparent. More recently it is used in two different ways. Sometimes it is used as you suggest meaning semitransparent, allowing some light, but not all, to pass through. For example in the Wikipedia article on Opacity.
An opaque object is neither transparent (allowing all light to pass through) nor translucent (allowing some light to pass through).
I have seen also seen it defined this way in high school science texts. More often, at least by my judgement, the translucency of an object is describes the degree to which light is scattered as it passes through it. In this usage it is not directly related to transparency.
Gammarist.com explains the difference like so:
Things that are transparent are so clear you can see through them as if there’s nothing there. Things that are translucent allow light through but with significant diffusion or distortion.
The Wikipedia article on Transparency and Translucency says essentially the same thing but with more detail.
In other words, a translucent medium allows the transport of light while a transparent medium not only allows the transport of light but allows for image formation. The opposite property of translucency is opacity. Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color.
If it were up to me translucent would only be used to describe substances that scatter light as it passes through but that's not the way everyone uses it.