The coat of arms of Simon de Montfort the Earl of Leicester is a red lion rampant on a white ground. But depictions of his arms almost always show a white lion on a red ground. The error is quite understandable. In the cathedral at Chartres there's a window depicting Simon de Montfort, and there his shield shows a white lion on a red ground.
The problem is that this portrays the Earl of Leicester's father, Simon de Montfort, the great crusader in Palestine and against the Albigensians. Financial contributions for Chartres' windows were being collected at a perfect time for this hero of militant Christianity to be so honored. There is no conceivable reason why England's revolutionary Earl of Leicester should be honored with a window at Chartres.
As a younger son, Earl Simon would have had a shield "differenced" from his father's -- as indeed it is in showing the colors reversed.
The proof of the Earl's red lion on white is absolute. Matthew Paris, who knew the Earl well and published both private letters of his and private conversations in that most reliable of all medieval English chronicles, the Chronica Majora, devoted an entire page to the depiction of the shields of England's lords. Simon de Montfort's shield is shown with a red lion on a white ground. And, should there be any quibble about the writing of the name beneath the shield, the 13th century illustration of Simon's dismemberment at the battle of Evesham should leave no doubt whatsoever -- for the shield with the red lion rampant on a white ground is depicted right beside the dismembered body.
It should be noted that the lion is a European lion, similar to a mountain lion or cougar, not an African lion. The difference is that the European lion was smooth-coated without mane, "feathering" of legs or a tasseled tail.
There are beautiful stained-glass windows, toys, and emblems of Simon de Montfort, intending the Earl but showing the Crusader. It may be too much to hope that what is out in public circulation can be changed, but perhaps future use of the Earl's heraldry actually will show that of the Earl.