The reason for using oil based inks is so that when I handpaint with traditional artist watercolours the oil based ink of the printed area repels the watercolour. If you were to use a waterbased ink (which dries quickly and cleans up with water) then when you come to handpaint it also dissolves the water based ink. Also you get such a lovely lustrous black with the oil based ink which highlights the colour of the transparent watercolour which I also like.
I always start handcolouring by taking a less successful print (one with flaws such as overinking, underinking, missed patches etc) and use this as a ‘working handcolured proof. It is where I experiment with colours and make notes on the colours I have used. It is a working proof which means I record the colours used on the print (just by penciling them in on the border and sometimes painting a little square of colour) and note any changes I would make on the final editioned prints. That way I can remember what colours I have used, how I have mixed the colours & keep a record of the actual colour I used for matching later.