What is a protoype?
Many may be familiar with the word, but in the last decade especially it has often been misunderstood.
A prototype part is an unfinished version of a to be released part. To qualify as a prototype part, the mould has to be different from the released version. In other words it has to have a different shape, design, or markings. Just a different colour is not good enough, and this is where the word "prototype" is often misused.
Test parts
Parts that do not qualify as a prototype may qualify as a "test part". It is a more general label for all parts related to testing, this includes prototypes. It really is that straight forward, if a part was made for testing, it is a test part.
2x4 bricks have been used for testing since day 1. Some of you may be familiar with the name "Bayer bricks", made by the plastic manufacturer Bayer in Germany. These bricks often have letters on the studs instead "LEGO", but are indeed LEGO test bricks, made by Bayer on behalf of LEGO. Now you may wonder why these bricks are not counted as prototypes, since they are different from the released 2x4 bricks. The answer is quite simple, they are finished parts with their purpose being testing. A so called "test part"... :)
Q-elements/parts
This is where the line can become a little blurry. Because not all parts in unreleased colours fall into the category "test part", they may be a "Q-part". A Q-part is LEGO's name for a part in a colour that was never released in a set, but meant for model builders and designers. These parts are not made for testing, but for building, and only for internal use.
It can be difficult to differentiate a test part from a Q-part if you do not know it's origin. The colour is usually a giveaway, but does not have to be. For example, sand green is most likely not a test colour, it is more likely used for a model builder. Red, black and trans-clear are more typical colours used for testing. Also the kind of part could be a giveaway. Model builders tend to use mainly basic bricks and not a lot of speciality parts. So if you find a 1x6 brick in an unreleased colour it is likely to be a Q-part, while a minifigure headpiece is more likely to be a test part.
As mentioned earlier this is a blurry line, but more often than not a part in an unreleased colour is a Q-part rather than a test parts. Q-parts tend have an easier way out to us LEGO fans than the test parts kept inside secure areas. They even show up for sale in pick a brick shops in LEGOLAND from time to time.
This article is currently being written, more information will come...