I also had special colored envelopes, not just white envelopes. 

They aren't anything fancy (small 1-2 bedroom condos) but I keep them well maintained, nice appliances, nice paint colors, so they attract better quality tenants.  

For prospective home owners, it typically allows them to select some finishes such as paint colors, trim, flooring, and so on.These will typically NOT be a good investment property unless it will cash flow when purchased at full market value (possible, but not likely).

3) White envelopes or colored envelopes? 

Should I stain the piece of wood to some contrasting color?

I love well cut blocks, stones, chiseled statues, decorative windows and door steps, green lawns, shaped architecture, arches, fountains, complementing colors, columns and building trims.

We look for old formica, avacado color appliances, worn hardwood floors, etc.

Why do I like the color orange?

I love the colors.  

You can try to match the wall color as best you can, no need to paint the whole house, just the walls that need it.