You can use indexOf(). Like:
var Color = ["blue", "black", "brown", "gold"];
var a = Color.indexOf("brown");
alert(a);
The indexOf() method searches the array for the specified item, and returns its position. And return -1 if the item is not found.
If you want to search from end to start, use the lastIndexOf() method:
var Color = ["blue", "black", "brown", "gold"];
var a = Color.lastIndexOf("brown");
alert(a);
The search will start at the specified position, or at the end if no start position is specified, and end the search at the beginning of the array.
Returns -1 if the item is not found.