You can create a custom gradient by selecting the Gradient tool (G) and then clicking on the gradient thumbnail at the left of the Options Bar. To add a color, click below the gradient ramp and a color stop appears. Double-click on a color stop to open the Color Picker and change the color of the stop to create a blend with the adjacent color stops. Sometimes you may want additional stops that are exactly the same color. Rather than try to match the color of a stop, hold down the Option (PC: Alt) key and drag out an exact copy of the color stop.
This tip previously appeared in Colin Smith’s Photoshop Tips column, in the June/July, 2019 issue of Photoshop User magazine.