Koen, Sam,
The Voynich botanical palette (in fact the entire palette) avoids anything in the range pink-to-purple-to-black. As a rule a flower naturally more pink will be coloured red and anything towards the darker purple-to-black will be coloured blue. In some cases, where a flower or part of a plant is naturally coloured purple-to-black, that part may well be just omitted. This is a custom seen in earlier Egypt - from which the western herbal tradition arose (believe it or not). An example is Dracunculus vulgaris where the sheath is omitted in the 'botanical chamber' of Karnak and in medieval herbals, too. It is also omitted in the Voynich image, but unlike the European herbals, that 'tabu' or restriction is observed throughout the Vms. Another case of the purple sheath being omitted is in the 'banana plants group' image. (f.13r). I have also noted a similar habit in earlier India (c.1st-2ndC AD) but that might be incidental.
I've noticed that a common reaction to information about the palette is an immediate suggestion that "maybe the painter had no pink/purple/black paint" but of course anyone could mix a little red pigment to white, or add blue to red or mix up green, blue and a bit of red to make a fair black. The original makers evidently felt such colours were not to be used - not a latin European habit, but interestingly one which was preserved by the fifteenth century (or later) painters here.
In addition - and I know this will be 'take it or leave it' no matter how many folios I've explained in detail: none of the images I've looked at in detail has proven to be a portrait of just one plant.
They are more like a still life without the vase. Except - that there are limits to the groupings. Each group depicts plants that occur naturally in proximity to one another, and which have traditional uses which are equivalent (as with the Musaceae) or complementary (that is, the way that a plant yielding a fibre will be habitually complemented by one which was traditionally used to dye that particular fibre).
If a Silene is meant, the significance of both red and white petals, I'd expect, would mean that either the white-petalled or the blue-petalled would do.. whatever the thing was. Just as example.. S. succulenta was used to make soap. And since any fibre-plant and any dye-plant will need the cloth and/or the dyer well-washed at the end of the process, it might be reasonable to add an image of that Silene in the group.
Thinking of these as a kind of shopping list: "buy the fibre/cloth and don't forget to buy soap, too -either the white-flowered or the red-flowered kind"is the general idea.
Or you can look at Latin herbals - more restful, anyway.