Diagonal drawing: Select the brush (or pencil) tool → hold down the shift key, click at one end of the canvas, and then click at the other end to draw a diagonal.

Diagonal region fill color: new layer

→ Select Pen Tool → Select Selection → (After selection) Press ctrl+enter, and a dotted selection will appear → (Set the foreground) Press alt+delete to fill the color. If the diagonal area has been prepared before, then hold down ctrl and click the small box in front of the layer, and a dotted selection will appear, and then press alt+delete to fill the color.