The International Fuel Tax Association (IFTA) is very similar to the IRP, except it deals with fuel taxes rather than road taxes.
All apportioned vehicles must have valid IFTA stickers when they travel, to indicate that the proper fuel taxes have been paid to their base jurisdiction.

The lower 48 states of the USA and the 10 Canadian provinces are members of the IFTA - but the District of Columbia (Washington DC) is not.

The IFTA existed as far back as 1983, as an experimental agreement between Arizona, Iowa, and Washington.
In 1991, IFTA Inc. was created, and a law was passed that all jurisdictions must become members by September 30, 1996.
But some states were issuing stickers as early as 1994.

Detailed history of IFTA can be found here.
Click here to go to IFTA's web site.

Each year since 1994, a pair of stickers is issued - one sticker is placed on each side of the vehicle, usually around the doors.
The colors of the stickers for each year are listed below:

1994,1999,2004 Green
1995,2000,2005 Red
1996,2001,2006 Dark Yellow/Brownish
1997,2002,2007 Blue
1998,2003 Pink
2008,2011,2014 Green with White Border
2009,2012,2015 Red with White Border
2010,2013,2016 Blue with White Border

Here is a run of IFTA stickers from 1994 to the present.

Below are various IFTA stickers.

Below is a collection of sample IFTA stickers from the IFTA web site.

They do not update these consistently, so many different years can be seen.
(Looks like the most recent is 2013...)

Before the IFTA, each state had their own distinct fuel tax (or road tax) stickers.
Here are some various older fuel tax, vehicle tax, and motor carrier type stickers from the pre-IFTA era.