In this example I will create a diagonal line pattern, but this method can be used for any tessellating background pattern in Photoshop.
Go to File > New
Choose Width: 5 pixels
Choose Height: 5 pixels
Resolution: 72 pixels / inch
Color Mode: RGB
Select the Marquee tool by pressing Apple M (ctrl M)
Choose from the Application bar at the top of the screen
Style: Fixed size
Be sure to use the measurement type you are using (px, mm) alongside the value as this will automatically revert to centimetres if you only add a numerical value.
Click onto the canvas in the top left hand corner. (Apple + or Ctrl + is zoom if the canvas is appearing too small to see) If the square marker appears elsewhere on the canvas you can click again with the mouse into the corner or use the arrow keys to move the marker to the top left corner.
Choose the paint bucket from the toolbar (Apple G / Ctrl G) and fill the area highlighted with the marquee tool by clicking inside the dotted lines.
Move thedotted line to the right and down one with the cursor keys and fill the new area. Continue this process until you have a 5 square diagonal line.
To deselect the marque tool at the end – I find it easiest is to select the lasso tool (Apple L / Ctrl L) and click once on the canvas.
Once the marquee is deselected we need to save this pattern as a photoshop pattern so that it can be used to fill large areas and so it can be used again and again. To do this choose: Edit > Define Pattern
A pop up box will appear. Give the pattern a description that you can recognise if you are going to use this feature a lot. Click OK.
The pattern is now saved.
To try this pattern out. Create a new document 400 pixels by 400 pixels. This time with a white background.
Choose the paint bucket tool. Select pattern from the applications menu and then find the swatch that you just created from the dropdown menu.
Create a new layer by opening the layers pallette (F7 or windows > layers. Click on the icon to the left of the bin on the bottom of the panel to create a new layer.
Make sure that layer 1 is selected and click the paint bucket onto the canvas. You should now have a tessellating pattern.
To change the colour of the pattern – double click Layer 1 in the layers pallette to open the layer style panel
Choose colour overlay from the left hand menu on the panel.
The default is always bright red! Double click the colour swatch to open the colour picker and choose the colour you want. Click ok to close.
To change the colour of the white lines. Create another layer in the layers pallette.
Drag this layer in-between the background and layer 1.
Make sure the new layer 2 is selected choose the paint bucket and choose foreground instead of pattern from the drop down menu in the application bar.
Select the foreground colour you want to use by clicking on the top left colour on the toolbar and choosing the colour in the picker.
Click the paint bucket on the canvas.
You can further change this colour by repeating the layer style process on the new layer, or by creating further layers of different colours above layer 2.
This process of saving a pattern ’tile’ so that you can reuse it to create a larger repeating pattern can be used in all sorts of ways, with vector shapes or images. The top and bottom of the tile must line up and also the left and right sides to create a repeat pattern that is seamless.