I am doing a heat map of the density of schools within a specific area of a city. Right now, I have this:
I would like to create a more formal map in which the white non-coloured aspects are coloured also, in this case, in blue. This will allow me to represent the map of the whole city in specific colours and in a more readable way. Is there a way to do this in QGIS?
Use the Heatmap renderer for your point-layer: it will cover the whole map canvas.
If you want to cover everything outside of a certain polygon area (city), render this polygon with Inverted Polygon renderer.
However, see the comment by @Ian Turton: yon can create such a heatmap - however, you should think about the basic concepts of cartography and heatmaps and reflect if that really is a godd idea in your case.
Screenshot: showing how to create the heatmap. The Inverted polygon renderer is used in a semi-transparent way (heatmap is pale outside the polygon) to demonstrate it's use:
There are many different ways to solve your question. One would be to follow the workflow I described for a similar question.
Another, more simple way might be to make an underlying polygon layer (or the canvas background) the blue color you want for no data values.
If you don't want the heatmap to extend outside to the polygons you could clip it, or (as Babel suggested) just put another version of your polygon layer on top with an inverted fill.
