You can use this approach with any shape / artwork to create your Affinity Photo star brush (with random colors no less). Go to the star tool, it is beneath the rectangle tool in the tools panel.
Set the number of points etc for your artwork, curvature etc. You may also like to convert it to curves via the right click command and then you will be able to manipulate the nodes of the vector to create even more unique vector designs. You may want to distort or warp the design, perhaps use filters on it and the artwork will then automatically be converted to a pixel layer (which is required for the preset)
Gradients work amazingly well with this, you can create all kinds of amazing artwork by using an artwork filled with a gradient. Sadly, the application does not offer many by default but please check out my pages on how to add additional stops etc to create a variety of swatches that can be used and added to the presets panel
The key thing is that it must be a pixel layer so if you have used a vector design, this must be rasterized. Go to the layer menu and then the rasterize command. If you have added a layer effect to it such as a drop shadow then you will have to deselect the fx option and then the design is fully rasterized.
You now have a pixel layer and go to the brushes panel and right side and create from the current selection.
You will now see the entry appear in the panel and you can right click the entry or double click it and edit the settings. You can set the size dynamics to the max (dynamics tab) as well as the scattering X and Y as well as the hue jitter. Rotation dynamics are probably not needed with this fairly uniform design, it makes no real difference to the end result.
Go to the general and set the size (probably 500 or so) and then set the spacing. If you select the paint tool, you can test the strokes out as you change the settings. Close the edit panel and now apply across the image.
You may wish to turn off the wet edge feature, a great idea but it does mean that if your stroke has this on that the strokes can be seen through and you end up seeing the greens and blue etc designs beneath the red and it is just doesn't look great unless that is the required result.
On completion, you may like to go to the layer menu and new adjustment layer and add levels, etc to tweak the colors.