I have collected some data of the performance of one Wi-Fi network with a position stamp. Now I can show the position where I have acquired the data on a \"local\" Google maps.
It is not exactly what you want, but you could take a look at the HeatmapApi.
There is a free version for max. 100 datapoints. The main limitation regarding your requirements is that you cannot adapt the color scheme. This feature is announced on their web page but without a date.
I tried the API in the last few days and it works as expected and promised. It is provided as web service and takes parameters in the format [lat, lng, value] to compute the heatmap.
I have found a solution on how to create a heatmap with Google Maps that works locally: heatmap.js.
I have used and works very well for my scope.
Thanks