Why did my SnO sample change color from black to light yellow in the desiccator?

[Viswanath]

I'm PhD research scholar, I prepared SnO sample of color black and kept my sample in desiccator some days back. But now it is converted into light yellow color. what might be the reason? any idea?

 

[Medicol]

Could it be the oxidation that has changed its color ?
http://en.wikipedia.org/wiki/Tin(II)_oxide