So, the MM says "A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range." How does this work if a monster's assumed form is doing something it's true form cannot. For example, if a dragon with the shapeshifter ability is standing in a medium doorway, what does that look like through truesight?

I'd describe it like the human shape looks like a portal through which you can see a huge dragon. Or as a full sized dragon projection that you can see radiating from the human form and you can simply see the projection as if walls etc aren't in the way.

Thank you. That question has been burning a hole in my skull for ages.

Tell your player that using Truesight gives them a slight headache after a while, cause like I bet stuff like that would. 

I'd probably describe it as the person with truesight seeing a human standing in the doorway while surrounded by an immense "ethereal" (i.e. transparent) image of a dragon with the eyes of the dragon looking directly at you if the person is looking at you. The color of the transparent image would match the type of dragon. Similarly, werewolves in human form would have an imposed transparent image of their hybrid or wolf forms (probably hybrid since it would tend to make more sense to me as a DM).

However, keep in mind that truesight only does what it says it does. It can't see through fog, magical or otherwise for example. It also can't see through magical effects that impose the heavily obscured condition unless called out specifically as magical darkness or an illusion. For example, true sight will see through greater invisibility or the darkness spell but will not see through a warlock's shadows of moil.

Quote from David42 >>

I'd probably describe it as the person with truesight seeing a human standing in the doorway while surrounded by an immense "ethereal" (i.e. transparent) image of a dragon with the eyes of the dragon looking directly at you if the person is looking at you. The color of the transparent image would match the type of dragon. Similarly, werewolves in human form would have an imposed transparent image of their hybrid or wolf forms (probably hybrid since it would tend to make more sense to me as a DM).

However, keep in mind that truesight only does what it says it does. It can't see through fog, magical or otherwise for example. It also can't see through magical effects that impose the heavily obscured condition unless called out specifically as magical darkness or an illusion. For example, true sight will see through greater invisibility or the darkness spell but will not see through a warlock's shadows of moil.

 I like this description, but I'd be careful describing the "dragon image" as "ethereal"...since truesight allows sight into the ethereal plane, you would want to make clear that what you are seeing is the true form of the creature, not some dragon lurking in the ethereal plane behind it.  While visually, I imagine any other way of describing looking at a transformed creature like that to be a massive headache, mechanically it just means that you know the visual appearance of the real creature in addition to its transformed state. 

One question I have would be whether truesight grants the ability to distinguish between a transformed creature (ie, via a creature ability, wild shape, or spell like polymorph, and an illusion cast around the creature (ie, through disguise self or any of the "image" spells).  I would say yes since it says you can detect a visual illusion, not just see through it, but I'm interested in others interpretations.

When applying that example with dragons in my games I would not let True Sight reveal the color of dragon.  True seeing/sight has limits.

"A creature with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceives the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the creature can see into the Ethereal Plane."

Truesight lets you see the true original form of a creature. Form  denotes the general shape,size,structure of the creature; not the coloration. As such, seeing a polymorphed dragon one could see the original dragon form of the creature, the size to guess its age perhaps, but would require perhaps scrutinization of anatomical form to determine the type of dragon through  some type of roll. 

More edge cases. A dragon polymorphed into a human that's completely covered top to bottom. Mask no skin to see. It's not going to tell you what color of humanoid they chose to polymorph into or their human specific shape was. I.E. True Sight was not intended to discern through normal disguises and it doesn't magically reveal anything about an their visible/presented form that you couldn't already determine with plain sight. I.E. sans an illusion it doesn't let you see through natural disguises clothes, etcs. It only gives you insight into the true nature...i.e. a dragon spoke to you.

That's specifically for the polymorphed case.
If they were shapeshifted via illusion you would flat out see through the illusion noticing their true color and form instantly. Since true sight is more like a magical filter to filter out all magical hindrances.

Personally, I would allow truesight to see the colour of a dragon that is polymorphed, as I interpret the word "form" in this case to mean "the way in which something exists" and not "the physical shape and configuration of something". Shapechanging changes more than just the shape of the creature, it causes it to exist in a different way, and so I would allow truesight to see the original form in that sense. It's definitely an interesting question though because it's really not clear cut at all what the intention was when they wrote that.

I agree with you. When you say that something or someone goes back to normal, you say it reveals its true form, not its true form and color. I don't think the book is using the firm dictionary definition of form here.

Quote from freekoout >>

I agree with you. When you say that something or someone goes back to normal, you say it reveals its true form, not its true form and color. I don't think the book is using the firm dictionary definition of form here.

 It could very well be, as there are 46 different definitions on dictionary.com.

Note that the word "perceive" is quite broad in its meaning, so you could picture it any way you want. You could indeed see some sort of semi-transparent projection, or you could have a strong and unique feeling that they're not what they appear and simply "know" what they truly are.

What I have seen of Dragons in D&D indicates to me that there is more that separates the visual appearance of the different dragons than simply the color of their scales. I would think that anyone with a suitable knowledge of dragons (and perhaps a successful ability skill check) would be able to discern the type of dragon from a depiction, even without it being in color. I know that doesn't really take a position on whether true sight shows in color, but in this case, I don't think it matters.