should i place the function (this) if i am adding to a list?

There is nothing wrong with it, this only means that you will have to call the methods by an instance of sequence class in order to generate the master sequence.

You can also create a getter and setter for the same or initialize the master sequence within the constructor.

public class Sequence
{
private var _MasterSequence = new List();
public List MasterSequence {
get
{
List colors= new List(){"Red","Yellow","Blue","green","Black"};
foreach(var (item, index) in colors.WithIndex() )
{
index%=item.Length;
_MasterSequence.Add(colors[index]);
}
return _MasterSequence;
}
}
// Sequence seq= new Sequence();
//var MS=seq.MasterSequnce;