Seriously. I want them for Franklin, by the way. Not the shorts. Not the cargo pants. Just pants. They exist according to this.
I am playing on the PC.
First off you will want to go to Ponsonbys (I went to this one). Enter and go to the back left wall(white with three suit models). Browse the suits and you will see that there is a beige suit, buy it. Now go somewhere that you are able to change clothes. Open the suit menu and select suit pants, you should then be able to wear your new pair of beige slacks. It seems that you aren't able to simply buy beige pants on their own, you must buy the entire suit. You will also not be able to wear the beige pants in some combinations of clothes such as with the boots for example (they will switch to a pair of jeans).
Note this is done on the latest update for GTA V.
At this Ponsonbys they have beige chinos:
At Suburban they have brown skinny jeans that look beige:
Same store also has these brown jeans that look beige:
Turns out beige pants aren't available in the PC version of GTAV. They're most likely just renamed tan pants.