Great idea! I gave it some though and my head just about exploded with awesome!
That can be done but it would need to fit with the ASCII simplicity. If i look at a door and see a chevron of a certain colour on it, that tells me right away the quality of the door.
For flooring, a + mark indicates smooth ground. If the colours are reversed it indicates engraved ground. With a little extra work you could add small indicators of only a few pixels:
1. The tile could have a coloured border. IE, grey square, black cross, gold box indicates a square of double detailed black stone with inlaid gold.
2. Single dots of colour at the corners of the tile. A black square with a grey cross and dots of green at the corners indicates smoothed plain stone with embedded green glass tiles.
The possibilities with this are endless!
1. You could have the cross indicating stone type superimposed by a smaller cross that covers the center point. Or the opposite with the tips of the cross coloured as an indicator.
2. The border of tiles could have different patterns of lines or shapes such as a "comb" or line of triangles.
3. The 4 right angles of the cross could be filled with triangles of colour.
4. Instead of just a dot of colour, the corners of each tile could be filled with triangles of other shapes.
If you make even a simple change to each tile, it forms a pattern. Triangles at the corner of each tile forms coloured diamonds when the tiles are repeated. All of this could be applied to doors as well. With even a few simple options we can form spectacular patterns for flooring, and make unique doors.
Unfortunaly this can not be implemented just yet. According to Toady there is no way to superimpose or display tiles on top of other ones yet. To do so would require a lot more coding.