Every .Net class supports the ToString() method. See if you can use it. But really the most appropriate way would be to get the class' type converter (the class here being Color) and convert to string. You would then use the same type converter to convert from string.
I was thinking something like this (in C#, but I bet you can convert it to C++/CLI):
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TypeConverter tc = TypeDescriptor.GetConverter(typeof(Color));
if (tc.CanConvertTo(typeof(string)))
{
//This is the string you need to save in your text file, and you need to read it back as a whole
//and pass it to the type converter again, except using ConvertFrom() instead of ConvertTo().
string strColor = (string)tc.ConvertTo(button1.BackColor, typeof(string));
}
NOTE: While I was trying to remember how to work with colors, I found two more classes of interest: A ColorConverter class (which might be the same as the type converter in the code), and a ColorTranslator class. They might also prove useful to you.