I am republishing a post by michael.ansel. I found a post by him to be the best way to change calendar colors.

One thing he failed to mention in the following steps is to install sqlite3 which is not installed by default. On your jailbroken iPhone, launch Cydia and install sqlite3.

Then proceed with these steps…

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If you have a jailbroken iPhone, you can SSH in and manually modify the colors in the SQLite3 database. Because the colors are stored simply as RGB values, you can also specify any color you want instead of being limited to the standard Red/Orange/Blue/Green/Purple.

Here are my notes for anyone else compelled to do this (its been working wonderfully for me!). One side note, you have to exit Calendar.app and then reopen it to see the color changes. Enjoy!

Disclaimer: If you even think about doing anything contained herein, your dog will die, your girlfriend will leave you, your phone will explode, and I will not be held responsible. I will however be glad to answer politely worded requests for assistance.
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$ cd /var/mobile/Library/Calendar
$ sqlite3 Calendar.sqlitedb

sqlite> select title,color_r,color_g,color_b from Calendar; -- List calendars and current colors
Default|-1|-1|-1
Michael Ansel|181|0|13
School-Important|47|141|0
School-Studying|15|77|140
School-Class|229|98|0

sqlite> update Calendar set color_r=181, color_g=0, color_b=13 where title = 'School-Important'; -- Red
sqlite> update Calendar set color_r=229, color_g=98, color_b=0 where title = 'School-Studying'; -- Orange
sqlite> update Calendar set color_r=103, color_g=10, color_b=108 where title = 'School-Class'; -- Purple
sqlite> update Calendar set color_r=15, color_g=77, color_b=140 where title = 'Michael Ansel'; -- Blue

sqlite> select title,color_r,color_g,color_b from Calendar; -- List calendars and current colors
Default|-1|-1|-1
Michael Ansel|15|77|140
School-Important|181|0|13
School-Studying|229|98|0
School-Class|103|10|108

sqlite> .quit

$ exit

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RGB Values for the Standard Colors

Red = (181,0,13)
Orange = (229,98,0)
Green = (47,141,0)
Blue = (15,77,140)
Purple = (103,10,108)

———-

Here are the values I use:

sqlite> update Calendar set color_r=181, color_g=0, color_b=13 where title = 'Important'; — Red
sqlite> update Calendar set color_r=229, color_g=98, color_b=0 where title = 'Work'; — Orange
sqlite> update Calendar set color_r=47, color_g=141, color_b=64 where title = 'School'; — Green
sqlite> update Calendar set color_r=196, color_g=168, color_b=131 where title = 'Health'; — Brown
sqlite> update Calendar set color_r=15, color_g=77, color_b=140 where title = 'Chris Carey'; — Blue

update Calendar set color_r=181, color_g=0, color_b=13 where title = 'Important';
update Calendar set color_r=229, color_g=98, color_b=0 where title = 'Work';
update Calendar set color_r=47, color_g=141, color_b=64 where title = 'School';
update Calendar set color_r=196, color_g=168, color_b=131 where title = 'Health';
update Calendar set color_r=15, color_g=77, color_b=140 where title = 'Chris Carey';