Sorry if my thoughts on this one are a little scattered...I was adding as I found them...I hope you can find something that works for you and grandma.
Good luck!
This is an interesting two tone pattern. I think it would look far more romantic in the colours you have on hand.
http://www.knitty.com/issuewinter02/PATTtangledgarden.html
You could also just start knitting your sock pattern and design your own pattern. Perhaps the toes and cuff one colour and the rest of the sock the other. The other option is to use your colours in stripes.
This pattern could also be adapted to two colours if you use one colour in place of the white and the other colour whenever either of the other two colours are called for.
http://www.straw.com/cpy/patterns2/socks/panda-wool-heart-shamrock.html
These illusion socks may be interesting too...I don't know your grandmother of course.
http://slippedstitch.blogspot.com/2005_10_01_slippedstitch_archive.html
Here is a site that teaches you how to make your own two toned patterned socks
http://tiajudy.com/cybersox.htm
What about either of these with the cuff one colour and the rest of the sock the other.
http://www.knitty.com/issuespring06/PATThedera.html
http://www.knitty.com/issuewinter06/PATTmonkey.html
This pattern shows striping in socks...I'm not so sure about the combo for g-ma, but you get the idea of how you could work with it in your colours.