So - I got to thinking about ArchVis (I know someone who does it), and what the client wants. When you show a “room in a house” to a prospective home-buyer, say the walls (in your 3D scene) are yellow, they might think: “Oh, I wonder how the room would look with a BLUE wall instead?”. What I wondered was - is it possible to let them do this? ie. click a button somewhere, pull up a color swatch and select a Blue colour from it, and have that yellow wall instantly become blue? …Is there any way to pull this off?? Please assume that the prospective buyer will not have Blender installed on the machine they’re watching on, or anything like that… If game engines are necessary, they can be used, of course, provided they’re not costly, oh, and the viewing hardware should be assumed to be pretty humble…
Is there any way?
You can connect blender up with Unity3d which has a rather neat web player allowing you to view the scene and interact with it inside a web browser. Shouldn’t take much to learn enough to script that.
In fact someone has already done it.
You can create such thing in Unreal Engine 4 (which is free for Arch-viz). You’ll need good understanding of coding or blueprint system though. Check this video:
What kind of hardware does that take to run??
…and is there any action from the free engines in this regard, like Armory3D…? 
Do any of them have a “web player”?
Ive messed with the player a few times and it adapts to the capabilities of the machine its running on to deliver the best possible experience. There are some demos here you can mess with, and there is even an archvis one.
Unity is “Free” in a sense via Unity Personal as long as your revenue is under $100K a year.