ive notice when attempting to manually vectorise raster images that... it can be tough to see where any one colour in the picture extends.
i.e. Due to variances in brightness across my LCD screen and interference to my perceptions due to different surrounding pixel colours in different areas on the screen the same colour in different contexts looks very different...
And can look totally different....
so i think some simple solutions to help visualise colours from raster images would be very handy.


A colour preview grid would be one....
This would be a special tool designed to help better visualise raster images when manually vetorizing them.
To use this tool the user would have to manually set up the palette with colours from the design that were of interest...
the Pressing a key on the keyboard (to call this function) at any time would bring up a small grid that would sit just north east of the cursor...
showing the 4 closest colours to the one directly under the cursor.
When the key for this function was held down this grid would remain and would follow the cursor maintaining the same colour choices taken when the button was first pressed...this way you could pick a colour and have the 4 closest colours from the palette held and then drag them around the screen to quickly see where these colours match up...

This is the idea in its simplest form... there would be many possible options to improve it....
Also i think if each box in this grid was numbered would be great as these numbers could also be superimposed over the palette to show which of these squares related to which palette colours.

But as a more advanced extension of this idea i think having this same tool have the ability to briefly alter the display would be cool too...
IE that whilst in this mode with the button depressed and the grid displayed...pressing the number for any of these colours would flash highlight this colour in the display....i.e it would operate on a display level...so that the user could quickly make sense of even a complex raster image...
AS when you pressed a number that related to a displayed colour in the grid it would flash (change) the occurance of this colour everywhere in the design area to a different very high contrast colour....Ie flash this colour on a display level to highlight it...

So when tracing a bitmap all you would do is first set up the colours you wanted as palette colours...
Then press the mini colour preview button at anytime to see which colour in the rester art was what.

P.S. i think this idea as a concept is a very useful one...if anyone has any suggested alterations or alternatives to solving the same issue i would be interested to know your thoughts so when i get around to proposing this the proposal is better developed.

P.P.S...
Another variation on this idea would be to simply take the last part of this idea and run with that.....
I.E ditch the grid all together and simply have a function to "Flash highlight" (briefly change) the current colour (the colour of the pixel currently under the cursor) to a high contrast colour so as to highlight this colour across the whole screen...
This would not so much be a vector thing but a display thing......i.e to help visualize complex colour scapes.

This tool could also have a sensitivity setting like the magic wand tool in most vector apps so as to adjust the sensitivity of the colour rage picked up by this tool.
IE a sensitivity of "1" would only pick up that one colour only where as a higher number would start to pick a wider range of simillar colours.

I think a way to help visual shapes of colours in a raster image has a lot of merit.

I think the feature you've described is perhaps overly complicated, and it might help to think about how people use aids to manually trace bitmaps already.

Basically, many vector artists will take their bitmap and posterise it. This reduces the colour depth and creates distinct regions of colour.

Taking that idea, Inkscape could apply a filter to the bitmap similar to posterise (given a set of parameters). The filter could be toggled on and off so the artist can easily work from both the original photo and the aid.

I can appreciate that you might still prefer your method where you can manually choose which colours should be used to define a region. VectorMagic allows you to select key colours that influences it's autotrace function, so I can see what flexibility it would provide. Perhaps as a part of the parameters for the filter a set of colours could be defined.

i can think of another possibility to really simplify my previous idea....
That is to incorporate this "Flash highlight" into the colour picker tool....

i.e. they could add in an option for the colour picker tool: "Flash highlight"

Which would, as well as displaying the colour of the pixel underneath the mouse pointer...also actively "flash highlight" this colour across the whole display as each new colour was encountered ......
This would be rather jarring to have different colours flashing across the whole screen as you moved ETC.... so perhaps holding a shortcut key in combination with the colour picker could be used to achieve this......