[5e] DM fiat dice rolls for roll20

yesterday i was DMing my Saturday group when an evil mage cast fireball out of a room and into a cramped hallway where the PC's had taken cover
i rolled 5d8=29 HP or DC 13 dex save for half damage. Rogue (6, crit fail) Wiz (11 failed) Cleric (7, crit fail) Fighter (in the room so was out of line of sight and range of the blast)

three of the four PC's where hiding in the hallway and failed their saves, the fighter ended up having to smack the mage down himself burning "Action Surge" and spending the majority of his superiority dice to down the mage as fast as possible, he lived with only 2 HP left. during the fight i could have used Shield spell to block the fighters hits but i wanted him to live so i didn't and basically ended up running a DerpMage so i didn't TPW us.
Later on after the fighter force fed some potions to the group he ended up dying when bugbears ripped him apart and he failed his first death save while the party was taking positions behind some crates and barrels. the next round he rolled a natural 1 on his death save... dying there on the spot.. i ruled that since they donated 50gp to the Shrine of Luck in the previous town that... that natural 1 was actually a natural 20... later that night the players asked if i had "cheated" and made up the fireball near deaths because i was "RL friends with the warrior" but then later saved my buddy when he was supposed to die.

what i want to know is there any way that i can fudge rolls with math to show a Crit success or failure ( the red/green border on [[ dice math ]] rolls) so i can throw the encounter in the players favor when ye olde S***'ith hit'ith the fan. or to influence events.

Nope and there's no intention for that to be added. Any dice rolled in chat are intended to be the same as if you rolled the dice on the table in front of the players.

bummer

If you've ever listened to the DnD podcast, they got TPKed a time or two. That just becomes a story element, like in the princess bride or other stories. The PCs wake up in a cell and have to find a way out... =D

If the players want to claim favoritism, the only way to avoid that is roll everything in the open. Some DM's do it all the time. I tend to switch to open rolling when I think the events have the potential to be deadly and don't mind letting the carnage happen. Most of the guys who've played with me for a long time have figured out they are safer when I'm rolling in secret. That seems to keep the accusations to a minimum.

Funny side note, your wizard did almost the exact thing one of mine did last time. That Fireball lobbed through a doorway is really deadly. Even worse is when they follow up with a Cone of Cold. :D

(@BGB: Ya bastard!! =D No more fights for my wizard with no spell slots... )

(and on a bit of a side note; you proooobably don't want to post in a public forum that you're trying to fake dice rolls for your players to see, if that's your intent)
If you literally just want to change the colors of min and max rolls, the only way to do that without the API is to make custom Rollable Tables and place colored images as the dice Pips however you want them.

/gmroll d20

I think this might do the trick It rolls the dice with the result hidden to the DM, and you can announce whatever result you want to interpret from the hidden roll.

Set it up as a macro like this,

/desc You hear the DM rolling dice!
/gmroll d20

More info on the Wiki,
https://wiki.roll20.net/Dice_Reference#Rolling_in_Secret

I'm honestly glad that you can't fudge dice rolls in Roll20.

I'm always having a weird feeling when the GM rolls everything secret and hits surprisingly often and makes his saves all the time, or at least at crucial times. It's quite possible he's totally honest about everything, and just rolled well, but it still leaves a little bit of an aftertaste.
The same when I GM. Sometimes it's needed to roll secret, but whenever it's not absolutely needed I let the players roll themselves.

So I'm glad that I know that when a diceroll is public, it's the real roll and not cheated in any way.

Quatar, if you read the original post in the thread, the DM wanted to save the characters from dying. He wasn't trying to hit surprisingly often. It's the opposite of your reply.

May I kindly suggest that we all make an effort to leave playstyle opinions to a minimum and focus on the mechanics in question?
Everyone plays different and no way is better than another.

/gmroll will only display the roll to the player and the GM. If the GM does the roll, they're the only one who sees it.
Same with /w GM [[ math ]]

Changing the min/max color coding of rolls in Roll20 takes the API or Rollable Tables using images with your own color coding.
Without API, there's no way for a player to trigger a roll that only the GM sees.

Indeed, let's not get into a playstyle debate, please. :)

I use GM rolls most of the time, I will modify damage sometimes, and mark it on a post it, to be given later in the fight. The fighter may very well die, but later add it to rolls, the fighter dies perhaps as the foe is reduced to a couple of HP and the rest of the party can finish it. This can often avoid a TPK and even the dead fighter feels good about it when he hears of his character's fighting to the death. If you have the API "blind roll" will allow any roll to be seen only by the GM. I use "blind roll" for thief skills, the thief should not know that he didn't see a trap because he didn;t see it, not because there was none there.