Can I use a colour that is not in the picture?

Yes, and it is often right. The matte belongs to the destination rather than to the artwork. A blue mark going on a cream page wants a cream matte, and cream appears nowhere in the mark.

What does sampling from the edge actually read?

The part-covered pixels, the ones under about sixty percent. Each of those is a mixture of the artwork and whatever the artwork used to sit on, and the artwork half can be read off the nearest solid pixel. Subtract it and what is left is the old background. Averaging the result over every such pixel gives a proposal rather than a certainty.

Is there a matte that is always safe?

No, and a tool that told you otherwise would be saving itself a control at your expense. White is the most common right answer, which is a different claim.

Choosing a matte colour

A matte is one flat colour under the whole frame. Four ways to pick it, and what each one is for.

The same mark on four mattes

Below is one piece of artwork, drawn four times, on the four colours the converter offers. Each panel sits on the kind of page it is meant for, so the question is not which panel is prettiest but which one stops being a rectangle when you look at it.

White on white

White on a tinted page

Page colour, matched

White on a dark panel

The four, and when each is right

White

The default, for two reasons. Most destinations are white — documents, forms, print, the middle of most web pages — and a wrong white is recoverable in a way a wrong black is not. A logo on an unexpected white field still reads as a logo. A logo on an unexpected black field reads as a mistake.

The colour of the page it lands on

Use this whenever the destination is tinted. Flatten a mark onto pure white, place it on a warm grey page, and the JPG announces its own edges: there is a bright rectangle around the artwork that was never part of the design. Matching the matte to the page makes the rectangle disappear even though the file is still rectangular.

Sampled from the edge

This one reads the picture rather than the destination. A part-covered pixel holds the artwork mixed with whatever it used to sit on; take the artwork half out, using the nearest solid pixel as the reference, and the remainder is the background somebody removed. Choosing it does not clean the halo away. It removes the halo by agreeing with it.

A value you type

For the case where you already know the number, which is more common than it sounds: a brand colour, a slide background, the exact grey from a stylesheet. The field takes three or six hex digits and the swatch beside it updates as you type.

Four things that make the choice easier

Decide where the file is going before you decide what colour goes under it. The matte belongs to the destination.
If the artwork is light, do not use a light matte, and the reverse. A matte that matches the artwork deletes the artwork.
If you are not sure where it is going, that is a sign the file should stay a PNG for now.
If the picture has no transparency, none of this applies and the swatch does nothing. The tool says so under the picture rather than letting you experiment with a control that is not connected to anything.

Three questions about the choice

Can I use a colour that is not in the picture?: Yes, and it is often right. The matte belongs to the destination rather than to the artwork. A blue mark going on a cream page wants a cream matte, and cream appears nowhere in the mark.
What does sampling from the edge actually read?: The part-covered pixels, the ones under about sixty percent. Each of those is a mixture of the artwork and whatever the artwork used to sit on, and the artwork half can be read off the nearest solid pixel. Subtract it and what is left is the old background. Averaging the result over every such pixel gives a proposal rather than a certainty.
Is there a matte that is always safe?: No, and a tool that told you otherwise would be saving itself a control at your expense. White is the most common right answer, which is a different claim.