1 - a cover letter doesnt need to be something fancy or formal. Your name, spouses name, we got married (date) at (location), and submitted our I-130 petition for spouse visa. Please see attached I-129F K3 petition.
2 - if it's not a huge hassle to do colour copies, I would just make colour copies. Most copy stores have colour copy machines.
3 - the only original thing needs to be the petition that you signed. You only have a copy of your marriage certificate - where is the original? I hope you didn't send an original with your I-130 because you will likely not get it back.
I presume that you understand that the chances for a K3 are extremely slim - maybe only 4 have been approved worldwide in 2023 - and why would you want one anyway, because then you have to do the lengthy/costly adjustment of status process which -- NOT having to do that -- is one of the key benefits of doing the I-130 in the first place.
I know there are some people who say that they feel it helps them "speed up" their I-130 by linking the two petitions together - but that's anecdotal evidence at best. For most people it does nothing at all - it seems very few it realistically helps. However, as it's free to submit, I guess there's no real "harm" in submitting it, but don't put much faith it will do anything in your favour.
