Re: use null or 0 in foreign key column, to mean "no value"?

​_My_ personal opinion is that NULL should _never_ be used to "indicate" anything at all, other than something like "?I don't have that information!"  NULL should be anathema because, as you have seen, just complicates coding SQL queries. 

What I have read (Joe Celko mainly) is that it is better to have "encoded special values" for "unusual" things. In your case, what does "no color" really mean? Does it mean "it has some color, but I don't know what it is" or is it more like a "transparent" substance, such as pure glass?​ Or is it "reflective" so that it is seems to have the color of what ever color of light is illuminating it? Maybe a perfect mirror of some sort. Are the "normal" color_id values always non-negative (0 or greater)? If so then I'd encode color_id something like -1 == "some unknown color", -2 == "transparent", -3 == "reflective", or some other meaning. One example that we actually have at work is in an "expiration date" on certain types of data files. Most of the time, this is just a normal date like 2017-01-20. But, sometimes, we want codes to indicate things such as "keep the last 3 copies, and expire previous copies" or "keep until it has not been referenced in 90 days" or "keep forever". We either have to have some "special" date values to indicate these latter non-date case, or we need yet another column for "type of expiration" which would be values for "use date", "permanent", "keep ??? days non-reference", "keep ??? copies" and so forth. We actually don't use an SQL date field, but a text field which we can test and, if necessary, convert to a date or to some "explanatory" text.