Obviously there's going to be variation between individuals as well as between photos so I wouldn't worry about it too much. If you want something more accurate you could try using the colour-picker pipette thingy on a drawing program then find out the RGB colour components. I'll have a look for a program which might put a name to the colour but you'll need to find a well lit, good photo as there's not really any way of doing that straight from the penguin's foot.
Just wondering, why do you want to know the colour of a Gentoo Penguin's foot? Knowing that might make it easier to give you an answer.
I hope that's helpful, but bear in mind that even if you were looking directly at a real penguin's foot the variation of light levels at that time would deny you ever getting an accurate colour. The only way I can think of doing it would be to get a penguin's foot, chemically remove the coloured pigment and run that, in a concentration identical to when it was still in the foot, through a colourimeter. But the penguin probably wouldn't appreciate that.