Suppose you have an image you want to put into an old TV or just apply similar effect to it.

The first step is to colorize the image. The unsafe way to do it is Hue/Saturation dialog (Ctrl-U, Colorize checkbox). But you will lose original image, and though you can always duplicate a layer to aviod losing the sources, I prefer to create new layer and fill it with solid color.

... and then change its blend more from "Normal" to "Color". This will make image from layer 1 use the color from layer 2.

Next, we need a TV grid. Create a new (temporary) image with 1 pixel width and 4 pixels height.

Use pencil tool or gradient tool to do an image similar to shown here. Then, select all (Ctrl-A) and define a pattern (Edit menu -> Define Pattern).