It would seem reasonable to use the coat tool as per the in-game tutorial to colour your objects and characters. However, this is not the most controlled way, and you can get a lot more out of the spraypaint tool instead.
Firstly if you coat an object it will override any spraypaint you do. Using coat is on a layer above spray, so if you coat an object first or after you will no longer see your spray paint. This can be confusing if you get the spraypaint tool and can hear the noise but nothing is being applied.My tip: Do not use coat at all unless you need to use its colour changing properties in some logic. If you accidentally apply a coat you can remove it in the sculpts properties menu.
Make sure you are scoped into the object you want to paint. The noise of the can will still sound even if you are not painting anything. Make sure the paint is actually going on, a good way is to have a good strong colour and a large shape so you can see the colour – then you can alter the blend, colour and opacity.
I like to use the sphere for spraypainting as this is the most similar to an actual real life can, but you can get good effects with the other shapes, especially if you are not using blend mode and want hard edges.
To change the tool from a solid colour to more of a dusting effect you need to change the blend mode. Do this with L1+square. Increase the blend mode and the effect will be softer and the colours will blend together rather than overlay. You can also use opacity to get a finer spray.
Using the custom colour tool is very good to get more subtle shades onto your object. Think about shading effects to create texture. A very good feature is the spinning palette. Grab a few colours and place them into the palette, making sure that you get “limbs”. You then use R2 to spin the blob. Now when you paint, the colours will change as the palette spins. If you use very similar colours the effect will be more subtle and great for variation such as skin or wood tones. Use blend to get the best out of this.
Dont forget you can make artworks with the spray tool on a canvas (just make a thin rectangle or square), it is not just for colouring your objects.Unlike the paint mode (which uses flecks that are physical objects), this is more like paint and adheres to the object you are painting.