I use Google Calendars a lot for schedulling, but the colors are never quite right. In fact, now I have all the different calendars as brown. There is no easy way to set colors, but if you have a jailbroken phone, you can actually hack SQLlite, or most simply, you just add calendars one at a time in the order you want. It takes time, but it works as Touchtip explains

Calendar on the iPhone will assign colors in a certain order for your first five calendars: red, orange, blue, green, and purple. Afterward, any more calendars are assigned a random color out of those five. To manipulate this, use iTunes to sync only selected calendars in the “Info” tab. First sync the one that you want to be red, then sync again including your second calendar that you want to be orange, and so forth. You can create extra calendars to use to “skip” a color if you wish, then delete later at your leisure for iOS 3.0
For iOS 4.0, 

In my experience, the iPhone (as well as the iPad) assigns the different colors to multiple calendars synchronized with GoogleCal via Exchange Server in the order you synchronize them. In other words, you have to synchronize multiple calendars step by step one after the other to match them with your desired colors. It´s a tedious and somewhat annoying procedure, but for me it worked. 
In general there are six different colors the iPhone/iPad assigns to the different calendars. They are assigned in the following order:
1. blue
2. red
3. green
4. orange
5. purple
6. brown
then the iPhone (iPad) starts over again with:
7. blue
8. red

However, I found that on my phone, this isn’t the color order, here is what I get on an iPhone 4 with Google calendar emulating Exchange and just adding one calendar at a time. There also appears to be a bug where the colors do change randomly and mainly end up as light brown. What a pain!

  1. Light Brown
  2. Light Purple
  3. light brown
  4. light yellow
  5. light green
  6. light red

I’m Rich & Co.

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