How do you add a fill or multiple fills to a symbol as well as adding a stroke or multiple strokes to a symbol, and adding swatches to either to the fill and stroke. So the key thing is to add the symbol to the artboard and then display the appearance panel which can be found via the window menu.
The symbol may contain multiple fills and strokes as well as effects and much more and you may find some symbols will not accept any additional fill or stroke etc but most will be fine with this approach. It is also possible that the swatch color will mean that fill is ineffective such as adding a black and white swatch to a black symbol will result in little change other than adding a swatch to the appearance)
To add to a symbol's fill etc
1. Go to the appearance panel in Illustrator and you will see a breakdown in the list for the symbol : symbol, contents, opacity.
2. Go to the right side menu of the symbol panel and then select the add new fill and the symbol will be filled with a solid black swatch.
3. If you only want to add a stroke then select the add new stroke command from the same menu.
4. OK, now go to the fill entry in the appearance panel and select the dropdown and then select one of the swatches you want to add to the symbol to color the artwork.
You will see the entries for the fill and stroke in between the symbol and contents entries. You will also see an entry for the opacity as well as the blending mode for the fill and the same for the stroke so instead of just using the normal blend for the color fill and the swatch, you can set the blending mode to lighten or darken or difference etc by going to the opacity and displaying the opacity panel and setting the blending mode there (some will have little effect and the stroke will have even less effect if there is nothing behind the design
The stroke can be added to the symbol and that can also use the same Illustrator swatch or a different swatch. Some combinations will work well and some will be gloriously garrish but it is up to you. The stroke also allows for the swatch to be manipulated a little further with stroke options such as caps and width profiles that can be added and you can access those by clicking the underline stroke entry
You can go back to the appearance panel and click the add new fill to add additional entries as well as add new stroke. The key thing here is that the entries added to the symbol are in order so any new fills and strokes added are on top of the originals and the original swatch will be lost unless you change the opacity or blending mode of the new swatch color. Likewise for the stroke but with the stroke the width of the stroke can also come into play so you can see the strokes added to the symbol if the smaller strokes are on top and the wider strokes are on the bottom (so if you have a 8pt and a 16pt stroke then you will see both of them but if the 16pt is above the 8pt then you will not see the 8pt unless you use blending modes etc).
You can move the entries in the appearance around at any time depending on the desired symbol / swatch combination and colors.
You can create some truly unique swatch fills and stroke color effects by combining the blending modes of multiple swatches especially using gradients but you can also combine the color effects also with the source symbol color to create perhaps subtle changes to the symbol instead of a radically different vector design
You don't have to keep the scaling etc for the swatch as defined,
1. select the entry in the appearance panel for the swatch / fill or swatch / stroke
2. use the object menu command and transform and apply a different scale such as 50% or rotate 45 degrees, though you should always remember to set the transform object to off and pattern to on (it should be a sticky feature but it always reverts back to object on when you don't want it).
All kinds of excellent fills and strokes can be added to the symbol just by using that feature to vary the result of the selected swatch
You can also quickly add effects as well to the stroke and the fill selected swatch and the modify the Illustrator symbol even further so just select the stroke or the fill and they can be added independently the swatch fills and stroke colors connected to the symbol.
The easiest way to access the effects is to select them via the bottom of the appearance panel and select the item you want to apply them to so select the stroke 1 and apply a 3D effect or warp or perhaps a third party color effect like Phantasm (Astute Graphics) and then select the stroke swatch 2 etc and then apply a blur to that and so on.
You can also just select the entire symbol and all the associated swatches and add them to the symbol panel and save it as a new artwork. You can then simply add new fills and new stroke swatches to that and start the process again. I would say, this must however have some impact on the system but if you have a powerful machine who knows how many combinations of color effects and symbols you can combine.