Almost a Ranger

Role: Victor Lee / Biorhythm Red

Role: Zack Taylor / Biorhythm Green

Role: Billy Cranston / Biorhythm Blue

Role: Trini Crystal / Biorhythm Yellow

Role: Kimberly Harte / Biorhythm Pink

Role: Trini Kwan (originally "Trini Crystal") / Mighty Morphin Yellow
How close was she? Audri filmed the pilot before the show got retooled and she was let go.
Why she was let go: She decided not to take the risk and drop everything for the show.

Trivia: If you look closely at the final version of Day of the Dumpster, you'll see a shot of Audri's Trini while everyone is outside the Command Center.

Role: Mighty Morphin White
How close was he? He won the part.
Why he was let go: VR Troopers (originally called Cybertron) was initially a project for Jason David Frank after the Green Ranger was written out. Then fans became angry once Frank was off the show and producers asked Brad if he wanted his own show instead, to which he obviously said yes.

Role: Adam Park / Mighty Morphin Black
How close was he? He had gotten far enough in the audition process to actually meet the actors who made the show.
Why he was turned down: According to reports, there were multiple locations for auditions, and the casting director didn't like the finalists from the other locations.

Role: Chad Lee / Lightspeed Blue
How close was he? Unknown, but he won the role of the Quantum Ranger the following year.
Why was he turned down? According to Dan, he was "too intense" for the role.

Trivia: Dan actually did portray Adam during the live show, Mighty Morphin Power Rangers World Tour Live on Stage. In fact, he pretty much became a regular suit performer for any live appearance, and wore several different suits over several years.
Dan also auditioned for yet ANOTHER character that he's never really talked about.

Role: Dulcea
How close was she? Unknown.
Why she was turned down: They thought she looked too young for the part. However, when Amy Jo decided to depart, she got a better gig on the show.

Role: Dulcea
How close was she: Mariska filmed several scenes. However, according to various cast members, the directer wasn't very professional and a lot of things were changed, scrapped and re-filmed.
Why she was let go: Her scenes were some of the ones that weren't liked, so they brought back the original actress, who had to drop out prior to shooting.

Role: Maya / Lost Galaxy Yellow
How close was she: She may or may not have shot an entire episode before Cerina Vincent replaced her.
Why she was let go: Unknown.

Trivia: Apparently they never re-shot some of the wide shots during the moon scene with Cerina Vincent, so Erin is still in the final version of episode one.

Role: Eric Myers / Quantum Ranger
How close was he? He was up against Dan Southworth.
Why he was turned down? He thinks it's because he was too "egotistical," but that may not be the official reason.

Trivia: Rick may have auditioned for an earlier season, as well.

Role: Unknown Pink Ranger
How close was she: She was the runner-up.
Why she was turned down: Specifics unknown.

Rumors say that this was actually Alyssa from Wild Force, but Summer has herself claimed that she auditioned for Pink.
Also, not only was Alyssa one of three characters recast, but at least three of the four final female cast members in Wild Force were picked from another character's set of auditions.
There is also the fact that her episode of Angel was shot in December and Wild Force had five shooting scripts by January (and Power Rangers shoots, on average, three episodes at a time), meaning either her rebound to Joss Whedon was insanely fast (but possible), or Summer in fact auditioned a previous year.

Role: Alyssa Enrile / Wild Force White
How close was she: Apparently she actually won the role.
Why she was let go: According to reports, Koichi Sakamoto demanded a character that his wife could double for, thus she was given the role of Kendall instead.

Trivia: Fox Kids actually released a promo with Sandra's voice over the White Ranger, but no live action footage seems to have been filmed while she was in the role.

Role: Princess Shayla
How close was she: Filming was already underway when she was recast and the second Shayla had a backlog of five episodes worth of scripts.
Why she was let go: Unknown.

Trivia: If you look closely at the storybook in episode three, the princess design actually resembles Natasha's portrayal of the character.

Role: Tori Hanson / Blue Wind Ranger
How close was she? Close enough that the people in charge remembered her.
Why she was turned down: She just didn't fit the role, but was brought back to audition for Kira the following year. And won.

Role: Kira Ford / Yellow Dino Ranger
How close was she? It was down to her and Emma Lahana.
Why she was turned down? The character was a singer and Anna had the weaker voice.

Role: Jack Landors / S.P.D. [B-Squad] Red
How close was he? Unknown, but the producer mentions his interview at around 4:30 in this interview.
Why he was turned down?

Role: Ronny Robinson / Overdrive Yellow
How close was she: The final four
Why she was turned down: Unknown.

Role: "Sadie" / Samurai Pink
How close whas she?
Why she was turned down:

Trivia: One of Eliana's earliest on-screen acting roles is a small, speaking role in Power Rangers S.P.D.

Role: Tyzonn / Mercury Ranger (Operation Overdrive)
How close was he?
Why he was turned down: Unknown, but he auditioned for R.J. the next year... and got it.

Role: "Kayla" / RPM Yellow
How close was she: Obviously she lost the role before the name changed.
Why she was turned down: It's not known if she did indeed win the role, or it was just wishful thinking on her and/or her agent's part.

Role: Unknown R.P.M. Character
Show close was she? Unknown, but came back and earned the role of Lauren the following season.
Why she was turned down: Unknown.

Role: Unknown Samurai Ranger
Show close was he? Finalist
Why he was turned down:

Role: Emily / Samurai Yellow
Show close was she? Finalist
Why she was turned down: She turned it down because she wasn't feeling it, but they called her to audition next season (albeit as Pink initially) and got it.

Role: Megaforce Pink
How close was she?
Why she was turned down: She was told that they had already cast a black actor, which we now know was the Blue Ranger.

Role: Calvin / Ninja Steel Yellow
How close was he? He flew out to New Zealand with the others to start pre-season boot camp.
Why he was let go: Specifics are unknown, but shortly after arriving in New Zealand he tweeted about going back to Los Angeles. Saban Brands later announced that, due to "unforeseen circumstances," the Yellow Ranger will now be played by Nico Greetham.