Dave,
You are in a situation that I lived with for 2 years. I have a magnolia wall and I used the Optoma H27, IN72 and HD65 with that. Honestly, the image looked great and I was very happy with it.
Then I took suggestion from this forum on installing a cheap Ikea Blind £19 from IKEA. It was only when I setup the blind I realised the difference in colours.
On doing a side by side, I noticed the colours on the wall was not right though I never realised that before. Our eyes can be tricked thinking everything is fine untill you change the reference.The blind resulted in much better picture.
I can suggest you two options
1. Paint the screen area with a grey colour as Kbfern suggested. or white if you have dark room.
2. Buy a cheap Ikea blind. It has two sides. use the sides based on your room environment
a. Whiter side - For best colour and brightness in a dark environment
b. Grey side - For better blacks if you have problem with light control
Remember the Ikea blind can only do a 90 inch diagonal image. For anything bigger, paint will be perfect.
You can vsist the HD65, IN80 and IN81 thread where I have posted many screenshots with the blind as the screen.
The Ikea blind can never match a good quality screen but it does a very good job for temp purpose or ppl who have issues installing a fixed screen like me.
hope this helps.
Ani