The alpha value of the image texture has been set to 0 so that connecting the "Image Texture" node's "Alpha" output socket with the "Fac" input socket of the "MixRGB" node chooses the "Color 1" input socket as the only color source for all pixels of the image texture until you paint on that image with a "Strength" setting for your brush higher than 0 (the brush's opacity => higher opacity can also be obtained with additional stroke over already painted areas while using a "Stength" value between 0 and 1 for the brush).