Convertors converting unconverted objects
Avoid long one-liners:

This: 

if ((int)value == 1) { return new SolidColorBrush(Color.FromArgb(255, 34, 34, 34)); }


Should be:

if ((int)value == 1)
{
    return new SolidColorBrush(Color.FromArgb(255, 34, 34, 34));
}


Typically one-liner if-statements are-so because they're short and can't be justified to take up 3 lines for something so small.

Avoid 'else' one-liners:

This is a clutter:

if (condition) { return val1; }
else { return val2; }


This isn't as big of a clutter, just space-consuming:

if (condition) 
{ 
    return val1; 
}
else 
{ 
    return val2; 
}


Acceptable alternatives:

I would do the following for maximum readability:

return condition ? val1 : val2;


Making use of the ternary operator and being the most maintainable/readable method.

In cases where you're not able to keep it one line, something like you had above is perfectly acceptable:

if (condition) { return val1; }
// Do something...?
return val2;


One-way converters:

I would consider the following situation before throwing an NotImplementedException().

public interface IOneWayValueConverter
{
    object Convert(object value, Type targetType, object parameter, string language);
}

// Optionally Adding "TwoWay" to the name (or some other similar naming method).
public interface IValueConverter : IOneWayValueConverter
{
    object ConvertBack(object value, Type targetType, object parameter, string language);
}


If this isn't an option (for any number of reasons) then it would seem the only acceptable answer to this situation would be throwing a NotImplementedException() as you stated.