Re: Problems on printing the color block after the keyblock
You have to wait a bit before doing the colours. A professional printer will do only the key block on the first day, then let the paper 'set' overnight, beginning the colour work the next day.
This will minimize transfer, but perhaps not completely stop it, especially when using a very light colour, as is the case with your light yellow. If the problem persists, you have a couple more options: wipe the colour block after each impression, to remove the grey residue, or dry the paper once after printing the keyblock impression, wait a day or so, then re-moisten and continue. That takes time, but will certainly stop the problem.
It might also be worth mentioning that when printing the keyblock on an ukiyoe-type print, a generally thin black is used, with as little paste as possible. Lines printed this way almost never transfer. If a thick black outline is desired these are (sometimes) done last, rather than first, with a lot of good testing initially, to ensure that the colour blocks do indeed line up properly. (We do all our little Chibi Heroes
prints this way ... key block last.)