Create a new canvas in Photoshop, we’ll be working on a 250 x 180 mm sized canvas in RGB mode at 300 dpi. This is important because the values for filters that work on pixel sizes will need to be amended if you’re working on a different sized canvas.
In Photoshop select the Gradient Tool (circled) and click the Gradient Editor (circled) on the Options Bar (Window > Options) to edit the gradient. Change the two colour values as in the screen grab, you’ll need to click the little colour tab and the rectangle marked ‘Color’. Enter the two values in the field marked ‘#’ in the Color window that pops up. Change to a Radial gradient in the Options bar and draw from the center outwards.