We appreciate the sensitive nature of this question.
A light brown (e.g., the color of coffee with milk) with no hint of a reddish tint is not a niddah color and can be disregarded. It sounds as though this is the case here.
Even if this color was a little darker, since it was found on your husband’s leg, which makes contact with a number of surfaces, and not elsewhere, we would assume that it could be attributed to some other cause and thus disregarded.
Please be in touch with any further questions.