It depends on your point of view.
It’s perfectly valid in CSS 2.1 when used on the elements available in the css2.1 specs: namely, :first-line, :first-letter, :before and :after. If you are supporting IE8 then you must use the single colon syntax for the above elements.
If you use css3 the new pseudo elements (that weren’t available in CSS2.1) then you should use the double colon syntax.
From the specs:
For compatibility with existing style sheets, user agents must also accept the previous one-colon notation for pseudo-elements introduced in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and :after).
Millions of pages were written using :after long before ::after came into being and they don’t all suddenly become invalid 