Well do you know term "pair of trousers". You may say: in his trouser pocket. Keep in mind that those were two pieces originally. See cowboys' leather trousers.
By form, this is not plural but dual (or whatever the term is) - marking a pair. As if each sleeve is one item (מכנס), and you have two. The dual form remained in Hebrew mainly as relics, and מכנסיים is used both as one of this piece of clothing (with the two sleeves) or as many pieces. But then in the last couple of decades people started saying מכנס for one piece of clothing (with the two sleeves). Educated people don't say it, and it sounds funny to me, but I do admit it makes much more sense; after all, one sleeve of pants is not a very useful thing to have a word for...
My hand dictionary (Dov Ben-Abba) says סְגוֹל is "violet." Is there a distinction or is it either-or? A website states: "Violet is a 'real' color that exists on the electromagnetic spectrum. It has a wavelength between 380nm and 450nm, sitting between blue and ultraviolet. When you split sunlight using a prism you will be able to see violet on one end. ... Purple on the other hand is a perceived color that is a mixture of red and blue." Source: https://www.quora.com/What-is-the-difference-between-violet-and-purple