Remove your torch immediately after you see the solder flow. If you don't, some of the solder could "burn away," resulting in no connection.
Copper bearing solder, when used correctly, really is not a copper color but a darkened silver color, which blends better with copper, especially after it is patinated. When heated too long after flowing, it turns more of a silver color. Also, if too much solder is used, it is more likely to flow away from the seam, creating grey solder flow areas around the seam.
It is best to remove excess solder, but this is not always possible. Sometimes the excess copper in reused pickle "coats" the solder when you pickle each piece, which can make the piece appear like the solder was never there, especially after using LOS to patina.
Your best bet when using copper bearing solder is to use way less than you think you need and immediately remove the heat when you see the solder flow.
Here's an example of Copper Paste Solder: