|1998||Home Alternate||Originally classified as the standard home cap, it was changed to a home alternate when the purple cap was introduced before the start of the first season. During that time, the logo was also reduced in size, though most of the versions that ended up in retail stores (like the one pictured here) still had the large logo. I believe it was only worn a few times with the alternate vests at home.|
|1998||Road||This was worn for all road games, regardless of the jersey.|
|1998||Home Alternate||I can only find one picture of this worn in a game, so it was worn very rarely.|
|1998-2006||Home||This cap was introduced after the original three, but before the start of the first season. It was also the only one that survived the first minor overhaul after the 1998 season. It was worn at all home games from 2001|
|2000-2006||Road||Though officially the road cap, this was also worn occasionally at home with the black jerseys in 2000 at least once during 2001.|
|2007-||Home/Road||Worn with both the home white and road gray jerseys, as well as the red alternates.|
|2007-||Home Alternate||Worn only at home with the black jerseys.|
|1999-2002||For the first time in 1999, MLB introduced caps made specifically for batting practice. Nearly all teams also wore them during Spring Training games, as the Diamondbacks have always done except for the first season in 1998.|
|2003-2006||A revolutionary material, designed to wick away moisture was created in 2003. The same material was also used in batting practice jerseys starting that year. The shape of the cap was a lower profile, and came in different sizes than the game caps. Instead of the standard 7 1/4, 7 3/8, 7 1/2, etc. sizing, the new caps came in S-M, M-L, and L-XL with a slightly elastic band to stretch for different fits. All teams used the same pattern, though not all chose to use a different color on the front of the bill like the Diamondbacks' purple.|
|2007-2009||The league-wide template was changed in 2007, coinciding with the Diamondbacks' new colors. The material and shape of the cap stayed the same, with the only change being the colored patches on the sides.|
|2010-current||Another new MLB-wide template change, with piping changed to go across the front of the crown and bill. This was the first red BP cap ever worn by the Diamondbacks.|
After 9/11/2001, all teams began wearing small American flag patches on the sides of their caps. In following years, they were also worn on the 4th of July, Memorial Day, and Labor Day. Beginning in 2008, a new cap was designed each year, with the flag design superimposed over the team's logo. In 2008 and 2009, all teams wore the same color cap, but starting in 2010, teams could chose either blue or red.
For the tenth anniversary in 2011, the original flag patches were worn instead of the newer superimposed logo design caps.
|2008||Worn over the 4th of July weekend for all three games against the Padres. All MLB teams wore similar caps. They were brought back for the September 11th weekend, with some teams wearing them only on the 11th, others wearing them for multiple games. The Diamondbacks did not play on the 11th, and did not wear the caps at all then.|
|2009||A new red design, otherwise identical to last year's version. It was worn on Memorial Day, the 4th of July, and 9/11. The red was a much brighter scarlet than the Sedona Red worn by the Diamondbacks.|
|2010||With navy and red used before, white was the natural next choice. However, this cap used a different colored bill, and for the first time, teams could chose between red and navy. Even though the red didn't match the rest of the uniform, that was the obvious choice for the Diamondbacks.|
|2011||A white panel front was used, similar to the style many teams wore in the 70's and 80's. This was not worn on September 11th, as the tenth anniversary was instead marked with a small flag patch affixed to the sides of the regular caps, as was done back in 2001.|
Even though the white cap was rarely worn on the field, it was still used in many promotional shots, and it even made its way onto many baseball cards, espacially those of minor leaguers. The caps used were usually the first editions with large logos. Most hats in retail shops and those still available online are also the ones with large logos.
The caps worn in games had a much smaller logo.
For a few years after the 1998 season, manager Buck Showalter and other coaches continued to wear the white/purple caps during Spring Training, though they were no longer officially part of the team's uniforms. Interestingly, Showalter would later continue this when he managed the Rangers, as he would wear a red cap during the spring, while his players wore blue caps. The red had been phased out years before Showalter even was hired by the Rangers.
As manager of the Rangers, Showalter would again opt to wear a different and obsolete cap during Spring Training. This photo is from 2005. The Rangers switched from red caps to blue in 2000. Maybe Buck was trying to be "retro."
Though almost never worn during games, the alternate caps made many appearances in promotional items before the games began. This baseball card from 1998 shows the teal/purple cap with the purple jersey, a combination which I believe was never worn during a game.
There is also a rumor about a purple cap with a deal bill, thanks to this 2000 card of Omar Daal. Baseball cards can be a good reference, but they are not always reliable. Many players who switch teams during the offseason have their picures airbrushed into their new uniforms. I don't think that was the case here, as Daal had been with the team since 1998. Certainly pictures were available of him in a Diamondbacks uniform. Another problem is that most pictures are from the season before, but that is not always so. This card is from 2000, so the picture usually would be from 1999. If you look closely, the patch on the sleeve is the 1998 inaugural version. Plus, the all black cap with the snake "D" logo was used in 1999. What this all means is not clear, but likely someone at Topps adjusted the settings to make the cap appear purple, for whatever reason. As this is the only "evidence" of this cap, I am fairly confident in saying it never existed, except maybe as a fashion cap.
In 2007, MLB introduced a new design to the cap for all teams. It just so happened that the Diamondbacks had a complete overhaul that year, but for most teams the differences were almost negligible. The new caps were made from a synthetic material designed to let out more heat and wick away sweat. The traditional wool caps mostly collected sweat and could even shrink after a few strenuous days in the sun. Overall, the look didn't change very much, aside from the underbill, which was changed to black. Most teams had been wearing gray, though a few like the Angels and Mariners had already switched to black, claiming it shaded the sun's glare better. Another minor change was the MLB logo on the back. It now featured raised stitching, similar to the logos on the front.
Gray underbills had been used since the beginning for the Diamondbacks, but all teams switched to black in 2007.
Caps from 2006 and earlier (left) had flat stitching on the backs for the MLB logos. With the new design in 2007 (right), the MLB logo had raised stitching. The result was a thicker look to the white area of the logo. This was the case for all teams' caps.
Perhaps the new material will prevent the "classic" sweaty look from being seen again.
One cap that has yet to see the field, but is an official option for players, is the DownFlap Cap. First introduced in 2009, it is a thicker material than a regular cap, and has flaps that can cover the ears in cold weather. Obviously, this will likely never be worn in Arizona, but a late-season road game in Colorado could lead to the first Diamondbacks player to sport this look on the field.