Stan Lee chose a grey skin tone for the Hulk because he wanted a colour that did not suggest any particular ethnic group. The printers had problems with the grey tones, resulting in varying shades of grey, and even green, to appear in the issue. After seeing the first published issue, Lee changed the Hulk's skin colour to green.
The green skin colour was used in all retellings of the Hulk's origin story, and reprints of the original story were even recoloured for the next two decades, until The Incredible Hulk vol. 2, #302 (December 1984) reintroduced the grey Hulk in flashbacks. Since then, reprints of the first issue have displayed the original grey colouring, with the fictional canon specifying that the Hulk's skin had initially been grey.