Answer by moonboots for Animating expanding gradient pattern on object

You could do it this way: Create an Input > Texture Coordinate node, a Vector > Mapping node, a Converter > Serpate XYZ node, use a Color > MixRGB node to mix a red and blue gradient with a grey color, and use a b&w ColorRamp as a factor to create the horizontal gradient. To take Marty's comment into account, if you want the color borders to be sharp, use the Constant option instead of Linear in the ColorRamp.