There is no Transparent color code, but there is an Opacity styling. With this, you will probably want to set the color of the element and then apply the opacity to it by using the lines of code below:-
You can use some online transparency generator which will also give you browser specific stylings. So you really need to overlay any other elements. Additionally, you may also want to try using an RGBA color using the Alpha (A) setting to change the opacity as mentioned in the example below:-
rgba(255, 255, 255, .4);
Using RGBA over opacity means that your child elements are not transparent.