Designing a PWM controlled power supply for a 30W RGB LED
Here is the design process I would follow:

First, get an idea of the max total supply required: here 30W * 16 COB LEDS. This makes about 500W. We can easily find such supplies off-shelf, however is very difficult to design such an AC-DC power supply at a DIY level. So, we make it simple: use a single power supply for the whole project, that we buy from somewhere.

Regarding the current sources for each LED string: I personally don't like to make designs that waste too much power, and sometimes, the cooling requirements make it more complicated than actually using switching regulation. So I rule out linear regulation and go for switching, which, as a benefit, gives you more flexibility regarding the main supply voltage.

The requirement for the regulator of a single LED string are not so big here: about 36V, 300mA. We need 48 of them, so we have to make them cheap (I guess). Go to mouser/digikey, find buck LED controllers that can handle more than 40V, and more than 300mA. Order by price. You get a few of them (for example AP8802, ILD6070, ...) Check the datasheet for the external components required and the typical use case schematics. Ok, not so many external components (integrated mosfet), the only big one is the inductor. Check that they can be dimmed using PWM. Select the inductor based on the datasheet info, check its size and price.

Go back to the main supply. Decide its voltage, which must be between the max regulators supply and the max regulators voltage drop + max LEDs voltage drop. Size it more precisely (taking account efficiency of current regulators). Find it for cheap on some website.

Go back to the individual regulators: make the schematic, make the layout (use big traces for big currents, follow the rules given in the regulator datasheet and make it as close as possible to the reference designs you can find, and try to maximize dissipation of chips if they have a powerpad underside), buy everything, solder, make the firmware, enjoy !

And if you have a more specific problem during any of those steps, don't hesitate to post a new question...