Jaabir bin Samurah (Radhiallahu Anhu reports that: "I saw the Seal of Prophethood of Rasullullah (Sallallahu alaihi wasallam) between his two shoulders, which was like a red tumour (protruding flesh), the size of which was like that of a pigeon's egg".
various and different narrations regarding the size and colour of the
Seal of Prophethood of Sayyidina Rasullullah (Sallallahu alaihi
wasallam). Qurtubi has reconciled these by sayimg that the size changed
from time to time, as did the colour. According to this humble servant,
this reconciliation may also be possible, that in reality all these are
similarities, and every similarity is according to a person's
understanding, which is an approximate state. There is no disagreement
in explaining approximates. This is a more appropriate explanation.
Source: Shama'il Tirmidhi