Create a new layer in Affinity Photo and then go to the tool panel and select the rectangular marquee tool and then create a thin vertical selection from top to bottom in the document. You can vary the size, you can make it super thin or perhaps something wider.
Fill that with a solid color or use the gradient tool to add a black and white gradient and then change that via the color picker panel such as changing the first stop to green or red and the second end stop to blue or yellow etc. The colors will change for the current applied effect. Of course, you can also add other designs to this such as adding shapes etc or strokes to the design or use blending modes to combine multiple presets together to create a super extreme colorful design. Or perhaps combine the artwork with effects such as the maximum blur and noise to bring in some extra sparkle to the strip. Youtube video on subject
Now go to the brushes panel found in the view menu and studio etc and go to the right side menu and click and create from current selection and you will now have a basic stroke to work from.
Double click the entry in the panel and change the spacing, size etc and also go to the dynamics tab and set the rotation jitter to the max as well as the hue and saturation. You now have your stroke design.
Go to the tools and select the paint brush and apply the new colorful stroke to your artwork and fill it with this beautiful colorful confetti design.
You may not be happy with the colors of the design, you may decide you want it to be more yellow etc. Well, you can always go back and change things but a quicker way is to use an adjustment via the layer menu and new layer adjustment and some thing like recolor or HSL or if you want a black and white effect, go for the black and white. Even quicker is to go to the view menu and studio and the presets and select one from there Youtube video on subject