Scottish Ancestry ??

I understand that all Johnston's can trace their ancestry back to the Scots. I don't know how true this is, but I certainly have no recent Scottish links in my heritage.... not within the last 3/4 generations at least. That said, I have always had a strong passion for Scotland, and have been particularly interested in discovering that there is a Clan Johnston.

When I first discovered this I was excited, but my delight quickly changed to confusion when I found 2 Coats of Arms, 2 Crests, and 3 Tartans. Which are the right ones, and if they are all right, then how do I know which applies to me?

In the hope that some kind soul will be able to offer me some sound advise in this regard, I reproduce them here.

John P. Johnston (no relative ... I think) has written to explain that the standard and ancient are different/lighter colours because the dye's used were from berries and faded easily, while the new one, or modern one, was made when artificial dye's were created and held deeper/darker colours longer. Thanks John. 

I have also recently discovered that many clans, Johnston's included, migrated to Ireland. I do know that my Great Grandfather came from Liverpool and had Irish connections.