What has this anything to do with the presence of Celts in Iberia ??
If you, in the Great Britain have blondes and fair complexion it's not because of Celts, it is because of the Vikings and Germanic presence.
Pure native British were and are dark haired, dark eyed.
The current evidence seems to point out that fair pigmentation came with the Indo-Europeans, while Neolithic and pre-Neolithic populations of Western Europe were darker. In that case, both the Celts and the Germans would have contributed to lighter pigmentation.
Celts were themselves mixed with indigenous Europeans, so it's not always easy to determine exactly their true genetic influence. Germanic people also intermingled with the indigenous I1 people, but I1 were likely to have had a lighter pigmentation due to their northern habitat (or else natural selection for fairer types was strong soon after the arrival of the first Indo-Europeans).