As Jibesh said, put
Random ranNum = new Random();
outside the for loop
and this inside
string colorName= "pic" + arrayColors[ranNum.Next(0, 6)];
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CODE
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Random ranNum = new Random();
for (i = 0;i<10;i++){
string colorName= "pic" + arrayColors[ranNum.Next(0, 6)];
}
[Edit]Code blocks added[/Edit]