One of the advantages of the Holga 12 CFN is the built-in flash. Not just any flash, but one with four color modes: white, yellow, red and blue. You can do fun things with a flash like that.
In general, don’t use the flash that often. With the simple cameras and flashes I prefer, when you use the flash you can easly kill a photo by flooding it with hard light. A colored flash in itself is not necessarily a bonus either. It can all too quickly lead to an monochrome flood of color. Maybe fun at start, but I quickly grow tired of it. This is actually the main reason why I was disappointed at first when I got my Colorsplash camera.
But unlike the Colorsplash, you can make double exposures with the Holga CFN. And then those colored flashes do become fun. By shooting a few layers while flashing with a different color each time, a festive mix of colored layers is created. Especially in party situations, this works very well, as if you are recording the disco strobe. But a harsh camping trip also can also be improveved with some colors.
It works best when it’s dark. In theory, you could also flash in daylight and make multiple layers. However, with too much light, the different color layers don´t really come across.
A variant is advancing the film al bit between each layer you shoot. This will give you a more elongated photo in which the time passes in different colors.
It’s a simple trick that gives a spectacular result.