Add new Affinity Designer gradients ? Select any shape such as a rectangle and then go to the top toolbar and select the fill and then set the fill to linear or radial etc and the current selected artwork will then be filled with a colorful design filled with stops.
The default is a two stop set of colors and you can edit those by going to the gradient panel / fill in the top left and clicking on either of the stops and then changing the color say from red to green or black to yellow.
You cannot move these default stops but you can also create stops by double clicking on the line and you can then go to the color entry in the panel and change that from red to green to blue say. You can create very complex designs this way filled with multiple colors.
You can re-position them and you can also change the midpoint where the color becomes more like the first stop or more like the second stop.
You can save this preset set of colors by going to the move tool and then going to the swatches palette and the preset can be added by clicking the button to the right of the category in the swatches panel. Please check out the video to see how to do this.
Create a rectangle and apply some stops to the design, the basic one being gray to white in linear. Click on the picker preset along the top control bar and click insert (9 times). Go the first one and click color and set to white. Go to the second stop and click color and set to black and go to the third stop and click color and set to white and repeat. Go to the move tool and select that and then swatches panel and set category and click the add to library. You will now see you have a black / white etc repeating swatch which can be applied to any other shape.