The color will get diluted in your final product.
I vape unflavored and I mix mine in 500ml or 1 Liter batches, so it makes the math simple and I really do not need a calculator. I vape 18mg, so I just put 180ml 100mg into my 1 liter bottle (90 for 500ml) and then add the PG/VG which I just eyeball those because if they are slightly off it is no big deal to me. I do 25% VG / 75% PG, I buy 100mg VG, so because the 180ml nic (18% of the 1 liter bottle) is already VG I only need to add a small amount of VG to get to 25% and the rest is PG. If I end up with 30/70 or 20/80 from being lazy and eyeballing it, I do not notice the difference because it is so minor. The main thing for me is getting the nic correct, that is something you want to measure.
You may want to mix a larger batch, like 500ml the way I described. You would just add 30ml 100mg to a 500ml bottle (60ml to a 1 liter bottle) and then add your 80/20 mix.
I do not use flavoring anymore, but when I did I would pour the mix I made into a smaller 100ml bottle and then add the flavoring to that the first few times until I found a percentage I liked, starting low (if it recommends 5%-10% I would start with 5mg/5%) and then working my way up 1mg at a time (because you can't go back). I would give plenty of time after each increase so that my coil had the correct mix. Then once I found the percentage of flavoring I liked, I would then add the flavoring to the large bottle. I would use 100ml bottles as my smaller bottle, that way the math was easy to do to convert to 500ml or 1 Liter... 5x (for 500ml) or 10x (for 1 liter) the amount of flavoring I put into the 100ml bottle. Or if you use multiple flavors, keep the 500ml (or 1 Liter) bottle unflavored and mix your different flavorings into 100ml bottles.
Doing it my way is not the correct way, but it works for me and it is easy. One reason why it is not the correct way when using flavorings is that if you're adding 5% flavoring to an already made batch of 6mg, you are diluting it slightly so your final mix will be just a tad under 6mg, but I highly doubt you would even notice that.
EDIT: Also, you may want to try unflavored (I used to use tobacco flavors, but slowly migrated to unflavored which I now prefer). Your coils will thank you for it, my current coil is on day #106 and it works just as good as day #1.