If a cat is white, is it a tomcat? - briefly
The color of a cat does not indicate its gender. Therefore, if a cat is white, it is not necessarily a tomcat.
If a cat is white, is it a tomcat? - in detail
The color of a cat does not determine its sex. "Tomcat" is specifically used to refer to male cats, regardless of their fur color. Thus, a white cat could be either male or female, and if it is indeed male, then it would accurately be referred to as a tomcat. However, the term "tomcat" is only appropriate when discussing male felines, irrespective of their coat color.