Mostly a Shiva lingam is made from a granite stone that is abundant in India. Hence, its color is black, but it can have different colors depending upon the materials used. It can be white also if made from a crystal or mercury.
In the Shiva Purana, there is a story of a female named Chanchula. When she reached the abode of Lord Shiva and saw him, she described him as white (Shiva Purana, Ch. 4, 45).
“He was white in complexion like camphor and wore all ornaments. Besmeared with white ashes all over the body and clad in white cloth, he shone brilliantly.”
Also, in a famous Sanskrit shloka “Karpura Gauram Karunavataram,” Lord Shiva is described as white like camphor.
According to the Linga Purana (1.3.2), the foremost lingam is devoid of smell, color, taste, sound, touch, and attributes. In another shloka, it is mentioned as golden in color. But in many paintings, Lord Shiva is described as pale or dark blue. Some people who saw Lord Shiva in their dreams claim that he is blue.
If the Shiva lingam is a phallus, then why is it mostly black and not blue or white?
It could be because black is the color of the cosmos.
But according to the Brahma Purana and the Shiva Purana, worshiping the mercury Shiva lingam is more beneficial, which is white, points to the fact that it can be white also.
Therefore, we cannot conclude anything concrete based on its color.