General Screen Recommendations:
The Epson Home Cinema 705HD is a practical projector. It needs a wall, or a garage door, or the side of a building. Of course, that's what it needs, what it would like, is a nice screen. Not an expensive one, with a low cost projector, that just wouldn't make much sense.
For those not looking for a permanent setup, most will either project on walls, or a pull-down screen. Some will paint their walls with screen paint, such as Goo. (Goo, and others should work better than your current off white wall, but I still prefer a screen). Besides, this projector is not exactly the type that attracts enthusiasts ready to rip up their room, repaint walls, etc.
Decent pull down screens of fairly large size, start around $200! Actually they start out closer to $100, but do get one that is "tensioned" as those will last far longer as they are designed not to get wrinkles or waves (which are very annoying).
Yes, for those of you really into ultimate performance at this budget, you would likely choose a high contrast gray surface, to improve the black levels. Certainly there's no shortage of lumens, in any mode, so a gray surface is not an issue regarding brightness. That's especially true if you have ambient light coming from the sides, which means an HC gray screen will, by its ability to "reject" side ambient light to a significant degree, may give you an overall brighter looking image, compared to the surroundings.
In other words, enough brightness to light up that family or bonus room nicely, even with a moderate amount of ambient light.
As I have said elsewhere, that's where I primarily see this projector - family and bonus type rooms, though that's not to say you can't build a home theater around it!
In summary, if you have a serious side lighting issue, you can go with either the HC gray type screen, or a very high gain white, both will help with the side ambient light. And the really high gain screens, give you some serious brightness, that few projectors can match. For "normal" people, who just want to run out and buy a projector and screen, set them up in a non-dedicated theater room, kick back and watch a movie or sporting event, each screen type has advantages. So consider your room - especially where any ambient light is coming from, and screen size, and judge accordingly. Ultimately, you want a nice low cost screen. If you want to fine tune your screen selection, those are my suggestions. Basically though, the standard white matte screen will do in a pinch.