I don't think the color of nicotine is too much of a big deal, it turns slightly pale yellowish color or even darker yellowish when oxidized depending on how the supplier, seller, stores them.
Fresh nic should be colorless iinm. If the supplier, seller bought in large volume and then they rebottled it in a smaller volume to be sell here there is high chance the nic had been introduce with oxygen thus change the color a bit unless they have a sophisticated lab equipment to eliminates the oxygen completely when rebottling, which I doubt it.

I'm not sure if there is any seller over here that bought a pure nic (which is expensive) and diluted it themselves in PG/VG mix but I highly doubt it. I think mostly bought 100mg in large volume and just do rebottle.

Link below is not anything scientific, but have read anyway:
http://blog.v-ecigs.com/2014/03/what-does-color-of-my-nicotine-mean.html

There has been a report that the flavor might crack certain plastics, o-rings I'm not sure. Could be though.
No problem. Find some scale that has 0.01 resolution and has "TARE" function. Make sure it has long cut off time as well so you could take your time while mixing.
If you can afford it buy some high quality like American Weigh scales on Amazon, but it is expensive lol.
Mine is nothing fancy but gets the job done just fine, you can find it on Lelong or Fasttech.
Agreed and I might add a bit, go for 0.01 resolution as well.

I found 2 places for your TFA Key Lime:
The Flavourist
Facebook/DIYyourOwnFlavor
Here is the chemical substances found in TFA Ripe Banana they're using to "mimic" the smell and taste of a ripe bananas. Found it on their site.

NOTE - All concentrated flavors are made from these types of molecules, even "natural" flavors
contain the types of items you will see listed in this section.
For example, natural cinnamon that you bake with contains cinnamic aldehyde.
Natural vanilla extract contains vanillin.

* PRELIMINARY DATA *
We will post new data as it becomes available - these are approximations only and are not definitive.

COMPONENT	Percentage	Description
Isoamyl Isovalerate	>= 10%	green, fruity apple
Propylene Glycol	>= 10%	no taste
Isoamyl acetate	>= 10%	sweet fruity, banana-like with a green ripe nuance
Eugenol	>= 1% and < 10%	sweet, warm spicy clove with phenolic and woody nuances
Limonene	>= 1% and < 10%	citrus herbal terpene camphor
Butanoic acid, 2-methyl-, 3-methylbutyl ester	>= 1% and < 10%	fresh green apple and pear with a waxy skin like nuance
Neryl acetate	< 1%	floral, rosy, soapy, fruity, pear and tropical
Vanillin	< 1%	vanilla, vanillin, sweet, creamy, spicy, phenolic and milky
Pinene <beta->	< .5%	appears in this flavor as a component of a natural extract - piney,minty with spicy nuance
Geranial	< .5%	found in natural plant material
3-Hexen-1-ol, (Z)-	< .5%	fresh, green, raw fruity with a pungent depth
Citronellyl acetate	< .5%	floral, waxy, aldehydic, with green fruity nuances. fruity pear and apple like
Neral	< .5%	found in natural plant materials
Propanoate <isopentyl->	< .5%	sweet, banana, fruity, tutti frutti, apple, melon, tropical and pineapple-like

The exact percentage of each substances is the property of TPA, but you get the rough "idea" what they're using to get the flavor profile. Some of the flavors they carry might not have a break down list of components like above (mostly their DX version or newer flavors), perhaps they're not ready to make it public just yet but most of their flavors data are out there on their site.

Anyway, andyng38 you must have a very sophisticated lab equip. to be able to extract your own banana flavor from a raw banana. I find it too much of a variables at play to get it right, for example like you said the sugar content in the raw bananas itself.

I mean who would've thought, while we were mixing our own liquid, they're mixing their own more advance chemical substances just to nailed certain flavors down. lol The chemists at these flavorings company is impressive.