I have loaded some OSM data into a postGIS database, using imposm. Now using this data base, I computed a value associated to every point of the map (could be anything, real-estate average price or yearly rainfall...).

Now I wish to draw the map and to represent that information on it with colors. Ideally, I need to generate both a neat classical jpg image and also tiles to be displayed on a website. I'm looking for a nice quality output.

I'm not looking for a GUI tool, I would like it to run on a server.

I had a look at Mapnik, I saw GeoServer and QGIS mentioned as well, but being new to the GIS subject I feel a bit confused, as I thought coloring a map would be a very vanilla task, but it seems not.

What tool would you please recommend ?