Gray was a humble color, and could be had by using leftover dye from making black. Cistercians and the Friars Minor of the Franciscans wore gray habits.
Pink was easy to make, since madder root used for red or brazil wood could be used in a lesser strength or by soaking the cloth for a shorter duration. Medieval illustrations often show ladies wearing pink dresses, which makes one wonder if pink was considered a feminine color centuries ago. Men also could be painted in pink, but their hosiery, not upper/outer wear.
If you wanted to suggest purity or innocence, you went for white. This was not always a bright white, but linen looked white enough prior to dyeing that it qualified as white. Since clothes could get dirty, wearing white was a sign of being able to afford to not get "down in the dirt" as a farmer. In the 14th century poem Piers Plowman—in which we happened to find the earliest reference to a "Robin Hood"—of all the characters introduced, there are few descriptions of their appearance, except when white clothing is mentioned; it always indicates a person of purity or innocence.*
Brown or russet/orange came in many shades and can be produced by anyone with access to boiling water and walnut shells or bark.
And so we come to purple, the color of royalty; so much so, that "born to the purple" indicated royalty. Purple candles were reserved for Advent, the "coming of Christ the King." Dressing a king's favorite in purple, as Edward II did for Piers Gaveston, was a clear indication to all the court of the king's favor. A species of sea snail (Murex) was an ancient source of the color, but it took thousands to make 1 gram of purple dye. Its expense made it rare, and its rarity made it valuable, and its value made it converted by the wealthy and ultimately limited to emperors and the highest classes.
There has been lots of research into the history of dyes. Although I do not quote from it, the Innsbruck Manuscript of 1330 (translated here) has instructions on dying. Vassar has a bibliography on works on dyeing. And there are many websites devoted to medieval times that instruct on dyeing the old-fashioned way. For instance, the illustration above is found on one excellent site.
In the paragraph on ray, I looked for a link to explain the Cistercians, and discovered that I have mentioned them many, many times, but there's no "introduction" to them. Let me correct that next time.
*A little trivia about me: "Clothing Imagery in Piers Plowman" was once of the first papers I ever wrote in grad school at UConn-Storrs.
