You can create a square brush via the brushes panel (right side menu) and once you have that new stroke in Affinity Photo, you can now edit it and you can do that by double clicking the entry and displaying the editor. Set the size to greater than the document height (say 1600 pixels etc) and then set the shape setting to a very small value and also set the spacing to a low value. Go to the dynamics tab and then set the hue and saturation etc sliders to the maximum and then you have your straight line color stripe brush stroke. Apply it to the current layer. It maybe slow as it is a big brush. You can also create a new layer and add the designs to that as well as then re-size the layer by using the bounding box as well as duplicate that layer and cover the document with different sized designs. You can also add an adjustment layer on top of all these or applied to an individual layer such as using the HSL or recolor to create some color variation with your design. Effects can also be added on top of the artwork as well such as deform and mirror.
70 Designs PNG (V178) Gallery $7.79 (also GBP, etc) per set, download
150 Designs PNG / ABR (V32) Gallery
100 Designs PNG / ABR (V168) Gallery
100 Designs ABR (V1) Gallery