Did you see jewelry artists and experts Tom and Kay Benham's response to a community member on the Jewelry Making Daily Ask the Expert Forum, about soldering gold to silver?
A. We recommend you use hard silver solder to solder a gold accent piece onto a sterling or fine silver backplate. Hard silver solder has the least amount of copper content, therefore it is the best color match to the silver and there will be less bleed or dark banding if it spreads on the silver surface during soldering. We don't recommend using gold solder as it would be quite noticeable if it should spread out during soldering. Also, the hard silver solder is less expensive than 14K gold solder, especially if the gold piece being soldered is large.
You may wish to use a compound to block the flow of solder outside the solder area. Yellow ochre is the traditional solder block/stop, though today many prefer to paint water-based correction fluid around the area to be contained. Be sure to use the water-based product that is eco-friendly and that does not emit harsh fumes during the solder process.