Can white walls replace projection screens? The answer is yes of course it can, just like you can offer cheap gas to your beauty --- Porsche. It can also run, but you can't get her excellent performance!
The projection screen has special optical materials to improve its reflective properties, while white walls are not. If possible, you may try to use the wall and the projection screen at the same time to see the difference. You will find that the image on the screen is much better than the walls in terms of brightness, contrast, and color saturation. Meanwhile the sharpness of the image is also lost due to the effect of the wall texture. We can learn from the experiment that with projection screen, the image that projectors give will be much better.
 
One more important thing you're missing is the frame of the screen. The image performance would also be better is there is a fixed black frame around. So that means you may need to paint a  rectangle on the wall if you would like to use it as the screen.
 
If you do insist using walls as screen, we suggest you use white walls. Never paint you walls to gray or black if you wanna use it as screen. Let's think about this: the projector is constantly updated every year, just like our DVD, audio equipment, etc., but the high quality projection screen can be used for a very long time. If you ever cares about your visual enjoyment, a good projection screen is really necessary and you will find it do worth your investment.
 
If you want to try a projection right now, I would say: Xinjieke projection screen, never let you down.. ：)