Color can be used to *enhance* information/content. But, can't be relied upon to convey information.
A bright red display saying "reactor core meltdown imminent" is not acceptable -- it relies on the user seeing the "redness" as being significant. Instead, you add "DANGER" to the message text in such a way that *it* stands out in the same way that color would (to a color-sensitive person).
If it *adds* information, then it is being misused. Imagine if I annotated each part number with a description IN CANTONESE! It clearly "adds information" that isn't (immediately) present in the part number, itself. But, that information would only be available to an individual who could "perceive" cantonese!
Chances are, your resistors are drawn differently than your capacitors. And, they probably have a different designator namespace (e.g., Rxxx vs. Cxxx). So, if you chose to display resistors in red and caps in blue, you haven't ADDED any information to the schematic. You've just made it easier to spot R's and C's "quickly".
Likewise, presenting power signals in a different color. Or "busses", etc. You use color to REDUNDANTLY convey information that is already present on the schematic.
Stop signs have a different shape and color from yield signs. But, this is all redundant information. Look at the shape of the sign and you know its content. Look at the text on the sign and you know its content. Even the
*color* gives you a good idea of content (all cautionary signs are yellow).If color *added* information, then put a red sign of indeterminate shape on a stick and it would mean "stop".
They aren't "keywords" in the sense that you are (probably) thinking. Structured documents impose/enforce constraints on "what can be where" (within the document).
E.g., a "revision number" can only exist within a "part definition".
You couldn't, for example, have a part's *description* appearing outside of the context of the *part* itself.
Or, a "quantity" appearing at some random place on a part's list (quantities are tied to "line items" and only make sense in that context).
If you see a "number" printed on a page, you can't tell if it has been *tagged* as a "quantity", "revision number", or "part number" -- without looking at the actual *tags* that encase it.
OTOH, if you opt to *show* a particular tag in a particular color, you've redundantly conveyed that information in a different form. So, if a "line item" in a part list doesn't have a "red" number where the quantity is expected, I know that my "DTD" (the thing that defines the structure of that type of document) is defective -- since it shouldn't
*allow* anything other than a "quantity" to sit in that spot!
background),
That means I can't use that "tool" in any environment other than the one in which those colors "make sense". Imagine ONLY being able to watch movies on a display that had the right color capabilities, aspect ratio, physical size, resolution, etc.
spell words,
look
You would be surprised at how newbies do things!
Some years ago, I was involved in the design of a color printer. One of the questions we addressed early on was "what do we mean by 'color'?" Were we trying to print color photographs? Or, "marketing presentations"? Or, PCB artwork proofs??
The needs for each application domain are very different. And, the price points for each *target* differ widely!
I, for example, would much prefer having the ability to make C size drawings in "a few distinct colors" -- even if I have limited control over those colors (much like the pen plotters of old). A and B size drawings I would like to be able to reproduce in a greater variety of colors -- but, probably wouldn't care if those were "true" (to some calibrated norm) colors. Finally, I would like to be able to produce small "photo-sized" documents with *calibrated* color accuracy.
These are all consequences on the types of things for which I use those "hard copies". A professional photographer would have a different set of criteria. And, a different *budget* for each of them! (I'd rather compromise on the large format prints in favor of a new 'scope; he might prefer a fancier printer since he's already made his "investment" in his ideal camera body, etc.)
The other problem with "cheap color" is that it starts to lose its impact. You see things in red frequently and soon red gets treated as a different shade of BLACK! :<
There's a fair bit of art/science involved in the *effective* use of color. And "whitespace". And typefaces. And...
Have a look at the sales literature that ends up in your mailbox each day. Or the political propaganda. Or, the colors that businessmen and politicians wear. etc. These are all very conscious choices made (to influence and/or manipulate). Almost invariably with the viewer being ignorant of the effects these things are *designed* to have on him/her! :>