I have an imac OS X version 10.6.8 . I want to update it to the newest version possible but when i press software update it turns off the computer and the purple screen shows up and it stays like that. Can someone please tell me what can i do?

Obviously that shouldn't be happening, but in any case Software Update only updates the existing system, and you are using the highest version of Snow Leopard. If your computer is compliant with the requirements, you can upgrade to Yosemite which is available free in the App Store in Applications. You will only be able to download the installer if your computer can handle it. Otherwise the earlier system Lion is available in the Online Apple Store for $19.99 - you will get a code to allow downloading from the App Store, but check your computer's compatibility first.

The requirements for Lion are:

• Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
• 2GB of memory
• OS X v10.6.6 or later (v10.6.8 recommended)
• 7GB of available space

The requirements Yosemite are:

• OS X v10.6.8 or later
• 2GB of memory
• 8GB of available space

and the supported models are:

• iMac (Mid 2007 or newer)
• MacBook (Late 2008 Aluminum, or Early 2009 or newer)
• MacBook Pro (Mid/Late 2007 or newer)
• Xserve (Early 2009)
• MacBook Air (Late 2008 or newer)
• Mac mini (Early 2009 or newer)
• Mac Pro (Early 2008 or newer)

Resetting SMC could solve the problem.

How long have you had this Mac?

Has it had the 10.6.8 COMBO v1.1 update applied? https://support.apple.com/kb/DL1399

Do you have Microsoft Word loaded or other purchased apps? If so they will not work on a higher OS than what you have.

System Management Controller -> Resetting the System Management Controller (SMC) on your Mac - Apple Support