The effect of the fermentative addition of overripe seeds by-product (Pedro Ximénez white grapes, 3 g/L) on the phenolic composition and colour of red wines was studied by rapid resolution liquid chromatography (RRLC-MS) and Differential Colorimetry. Overripe seeds summited to postharvest dehydration by sun drying directly demonstrated were rich in phenolics such as gallic acid, epicatechin, and procyanidin B2 3-O-gallate, and hence a source of copigments capable to stabilize wine anthocyanins. The fermentative addition of overripe seeds led to Syrah wines with significant higher content of anthocyanins and procyanidins than traditional macerated wines, which had a positive effect on colour quality and stability. With respect to the colour quality, visually perceptible colour differences were found respect to the traditional macerated wines (ΔE*ab > 3), getting overripe seeds macerated wines with bluish hues and higher chroma values. Moreover, Differential Colorimetry demonstrated that the addition of overripe seeds induces lower colour modification in wines during stabilization and, in consequence, higher colour stability.