Im a beginner.and please forgive me if my question is dumb
I would like to know what red blue green color does in a pyro sim if i go to the plane mode.I am not talking about the guides.
I am talking about the 2d planes.Eg something that comes for velocity visualization

Does red means like something is burning? and blue means that nothing is burning?

It is just a mapping for the magnitude of the velocity. IE. Red being high velocity and blue low.

Exactly what Erik said.
All of this is controlled on the Smoke Object DOP's Guides > Velocity tab. It is here that you control how the streamers and colors map to velocity. It defaults to infra-red where blue maps to minimum and red maps to maximum within a defined min and max range (0-1 by default).

With everything at it's defaults, the blue to red mapping by default indexes in to velocity going from 0.001 to 1. This means:
Visualization is set to Streamers.
Speed (velocity) is what is being sampled.
Any values below 0.001 are not displayed.
Values starting at 0.001 will be rendered blue.
Values approaching 1 will be red.
Values above 1 will be red.


I use this all the time to inspect the velocity field. You can quickly get a feel for the magnitude of the velocity by adjusting the Visualization Range's maximum value. Here's how:
- Turn on Velocity visualization in the Guides > Visualization tab
- go to Guides > Velocity tab
- play the simulation forward to a frame where you wish to inspect the velocity field.
- adjust the Visualization Range max value from 1 using the Right Mouse Button slider while watching the velocity field in the viewport (you may have to turn off the display of density). When the majority of the red has faded off, look at your maximum value. That is what the maximum velocities at that frame are in your simulation.

This is very handy to know.

You can also visualize other components of the vel fields by adjusting the Visualization Type parameter menu. For the vel fields, Speed makes most sense.

@Erik @ Jeff Thank you very much
I also have one more question

what about the 2d planes that comes with density,heat,temperature?

Same thing as with velocity. Each of these fields has guide geometry associated with it with it's own tab to control how to visualize the guide geometry.

For example, you could visualize temperature as a plane that you can slide in each of the three primary axes or you could enable the smoke option to visualize the temperature field as smoke.

Treat the default visualization settings as just that: defaults. You can set them up however you want.

One example is with the fuel field. By default it is set to smoke coloured purple I believe. With this option, yes you can see your source but it is a challenge to see if the source is hollow (only the boundary has fluid) or is solid. If it is ambiguous, you can go to the Source visualization tab on the Smoke Object DOP and turn off smoke which then shows a plane. Now slide the plane so that it intersects the fuel field so you can see if it is just a fuel source on the shell or if it is solid.

@jeff thanks for the explanation
how about the colors of the plane? same as colors of velocity 2d plane?

Yes that's the 2d plane colors. You have different mapping preferences in the Visualization Mode parameter: infrared, white to red, grayscale, black body and twotone.

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If you want to get a glimpse in to how all this happens for you inside the various assets that you use in a simulation, look at the attached hip file. It shows you how to add a scalar field with visualization data to an empty DOP object. It is quite obvious that it's a DOP that builds the Visualization data. In the case of a scalar field, you use a Scalar Field Visualization DOP. Vector fields get as you would expect a Vector Field Visualization DOP.

Knowing that this is how things are done, if you discover there is a field in a particular DOP object that does NOT have visualization data, you can always unlock it, find the DOP creating the field(s) and add an appropriate * Field Visualization DOP to that field and then be able to see it in the viewport.

Obviously this is an advanced task and not a very common one at that. More for Power Users seeking a deeper understanding of how sims are constructed and evaluate inside Houdini.