You can customize certain color options by going to Informant settings>themes.

There are pre-set theme options listed first: Standard, Dark, Black Header, Blue, & New Light

The last option is Custom.   To set specific colors you would tap the triangle to the right of 'custom' and then choose edit.

The options you can change will be listed and you would tap the color square to the right of that to bring up the color editor.  

Once you choose all the options you want to change, tap the done option at the top of the screen.

Now you will also need to tap the 'custom' option when you are taken back to the theme settings in order to apply the new colors.

These colors only affect the background colors or areas indicated in the custom theme settings.

Go to your calendar view and tap the menu icon at the bottom right.

Choose visible calendars>tap the triangle to the right of the calendar you want to change>choose the new color.