Create a Grid

1- First make a new image.Now click the Channels tab:Window - Channels. Create a new channel.

2- Now make a little design that will be tiled for the pattern. For example, to make a grid, click the Single Row Marquee tool and make a single white horizontal line on the screen, then click the Single Column Marquee tool and make a vertical white line on the screen.

3- Now click the Rectangular Marquee tool and make a box around the design (for the grid, the centre of the intersecting lines).

From there, go to Edit - Define Pattern.Now deselect the marquee tool and clear the whole image (so it's black again).

4- Now click the Paintbucket tool and set the options to Pattern, as opposed to the default Foreground setting. Choose Pattern and select the pattern that you saved in the previous step:

5- Now click on the screen or click OK and see the new black & white pattern (grid) you just made. Now while you hold Ctrl click on the channel to load its selection.

6- Now go back to the Layers tab and on a new layer, with any colour you want, fill in the pattern. You can also use gradients to fill: