export DISPLAY=10.20.30.100:0.0

cd /mount/CD/11gR2/database/

. ./runInstaller

Starting Oracle Universal Installer...

Checking Temp space: must be greater than 120 MB.   Actual 1808 MB    passed
Checking swap space: must be greater than 150 MB.   Actual 76389 MB    passed
Checking monitor: must be configured to display at least 256 colors
    >>> Could not execute auto check for display colors using command /usr/bin/xdpyinfo. Check if the DISPLAY variable is set.    Failed <<<<

Some requirement checks failed. You must fulfill these requirements before

continuing with the installation,

Continue? (y/n) [n] n

User Selected: No

Exiting Oracle Universal Installer, log for this session can be found at /u01/app/oracle/oraInventory/logs/installActions2010-12-29_04-34-56PM.log

Not sure why this is happening.  I set my variable and also in putty i enabled the putty session to use X forwarding as shown below: