Hi Dimitry, The attached notebook does this. But, in brief:
In[10]:= {redData, greenData, blueData} = Transpose@Partition[Flatten[imageData], 3];
In[11]:= StandardDeviation /@ {redData, greenData, blueData} // N
Out[11]= {74.1906, 55.9707, 57.511}
In[12]:= Mean /@ {redData, greenData, blueData} // N
Out[12]= {165.726, 151.901, 141.397}
Attachments: