Canon S5Is
Version 1.1.0.0
Cannot post picture
Yes, Zebra is enabled
Without a picture, it's hard to know what you are experiencing. Do you mean the screen goes completely solid yellow, or are some other things visible?
In general, CHDK should not cause the whole screen to be covered in a solid color when you half press. If the entire scene is over or under exposed, the zebra could cover it. Bugs in the zebra code could also potentially cause something like this. So I'd suggest trying with zebra disabled.
For version, I meant
CHDK version. You see this in the CHDK splash screen or in the CHDK menu under "Miscellaneous Stuff" -> "Show Build Info"
If you use a recent version of CHDK 1.6 (the "unstable" development branch) you can take a screenshot within CHDK:
Go to "Miscellaneous Stuff" -> "Tools" -> "Screen Record"
Select "Schedule Dump"
You will be returned to the <alt> screen. Exit alt, and half press. This needs to be done before the time in "Dump Start Delay" passes. You can increase it if needed.
The dump will be saved in DCIM/100CANON/ named something like CRW_0000.JPG. Post the file here. You can zip it if it's too large for the forum. The file is not actually a jpeg, so normal image programs won't open it correctly.