As far as I know, there are no colour standards for wiring in regards to RS232 implementations, only pin assignments.
A lot of people (including myself) wire DB9 connectors to mimic a guideline colour scheme used for RS232 on RJ45 (EIA-561).
Code:
Blue RI
Orange DCD
Black DTR
Red GND
Green RXD
Yellow TXD
Brown CTS
White RTS
More information on this scheme can be found here: **broken link removed**
Having said that, the YOST serial device wiring standard has been becoming more and more popular throughout the years. I would say that it is as close to being a "standard" as anything else I have seen. It's probably what I'll eventually move to in order to avoid complications in the future. More information can be found here: http://yost.com/computers/RJ45-serial/index.html