Besides, “ink” can be used in informal English as a verb in “to ink a contract” which means “to sign a contract”. e.g. Meisuowei（混混姐）has just inked a big deal ！ -softface(探长) @枫下论坛 The Rolia Forum

工作学习 / 外语学习 / 【英语学习】：大家常常提到的要JJ＂白纸黑字写下来＂的英语怎么说？这么说： Ask your agent to put his/her commitment in ink！ -softface(探长); 2014-8-14 (#8932177@0) +1

People normally say "put it down in black and white". -jiage(北伐先锋); 2014-8-14 (#8932200@0)

Since blue and brown pens are already accepted in the court as well as black ones, I write both. -softface(探长); 2014-8-14 (#8932215@0)

And request for his/her signature in front of you！当面签字以免有诈！ -softface(探长); 2014-8-14 (#8932185@0)

据说有一种特殊的墨水，过段时间字就消失，合同成了白纸 -alaabracadabra(AlaAbracadabra); 2014-8-14 (#8932205@0)

Nothing says commitment like ink！指天发誓都没有白纸黑字写下来管用！ -softface(探长); 2014-8-14 (#8932187@0)

Put it in writing, how does this sound? -errands_boy(Errands Boy); 2014-8-14 (#8932237@0)

Jesus, I almost mistook it for "ED" -errands_boy(Errands Boy); 2014-8-14 (#8932273@0)
'Ask him to put his commitment in writing.' is fine. But 'in writing' is opposite to "by mouth; orally' , which presents the tone not so strong as "in ink" or "writing black-and-white" . -softface(探长); 2014-8-14 (#8932291@0)

"in writing" is an everyday expression in general, which means the acting of this phrase could be done in any way and with any writing instrument, like a pen, a pencil or even a lip stick. -softface(探长); 2014-8-21 (#8942577@0)
But "in ink" conveys much more than "in writing" in meaning, especially in regard to the tool the writer will apply -- a pen that uses ink instead of any other writing tools like a pencil or a crayon or even a lipstick which do not use ink in them. -softface(探长); 2014-8-21 (#8942595@0)
"in ink" is similarly the same as the Chinese saying “签字画押（set one's hand and seal to something）” ， “盖章签字（affix one's seal and signature）” and “摁手印（put one's thumbprint to a document）”. -softface(探长); 2014-8-21 (#8942611@0)
Besides, “ink” can be used in informal English as a verb in “to ink a contract” which means “to sign a contract”. e.g. Meisuowei（混混姐）has just inked a big deal ！ -softface(探长); 2014-8-21 (#8942617@0)

官网广告文字：Ask your Realtor to put their commitment in INK！“I will find your dream home.” Nothing says commitment like ink. Ask them to sign a B.R.A. -softface(探长); 2014-8-18 (#8937882@0)

探长颇为欣赏ＪＪ局高超的＂春秋笔法＂——既非常巧妙地教授消费者如何对付日益增多的先是承诺返佣后又耍无赖的JJ，又让JJ们无话可说。 -softface(探长); 2014-8-21 (#8937887@0)

这种 innuendo【说东指西】是英语修辞（Rhetoric）中极为高超的一种技巧。 -softface(探长); 8.18 13:58 (#8937895@0) -softface(探长); 2014-8-21 (#8942682@0)

原先的版本：Ask your Realtor to put their commitment in INK！“I will find your dream home....” And request for his/her signature in front of you！Nothing says commitment like ink. -softface(探长); 2014-8-21 (#8942672@0)

会数数麽？1.jiage (#8932200@0)；2.alaabracadabra (#8932205@0)；3. errands_boy (#8932237@0)；4.meisuowei (#8932269@0)；5.vwkiller(#8946363@0) -softface(探长); 2014-8-23 (#8946372@0)

I admire your achievements in IT and your linguistic intuition in English. -facade(探长); 2014-8-24 (#8947391@0)

用铅笔写下来也是 in writing 耶，但是很容易被擦掉的啊！所以啊，还是 in ink 来得严实。 -softerface(探长); 2014-9-15 (#8987876@0) +1

那谁，你倒是看看你自己说的话：(#8840571@0) (#8841804@0) ；我来外语坛不是为了学英语，而是纯粹为了打假，顺便帮人回答点问题。 -onlook(且放白鹿青崖间); 11:51 (#8841867@0) -softerface(探长); 2014-8-28 (#8955503@0)
发PM的几位，看这里。俺就不一一回复了。 -softerface(探长); 2014-10-8 (#9030942@0)

关于返佣：买房经济口头答应给返佣。请问这条应该写在合同里吗？是要求返支票还是现金？ -bestbestlife(平安); 12:52 (#9476151@0) -softerface(探长); 2015-5-27 (#9476252@0)

起！ -pinfang(品房); 2015-8-23 (#9624035@0) +1
起！ -houseful(_); 2016-7-8 (#10189906@0) +1
起！ -houseful(_); 2017-2-1 (#10585235@0) +1
ＪＪ局的＂红头文件＂： -houseful(_); 2017-2-5 (#10592079@0)
『觅巢指南』防火防盗防经纪！—— 这不仅是坛里共识，也是多伦多地产局给所有消费者最有力又最无奈的忠告！见TREB另一官网BraFirst .Ca： -houseful(探长); 5-16 (#10802188@0) -houseful(_); 2017-5-17 (#10805576@0)
白纸黑字 -reddust(hongchen); 2017-6-25 {28} (#10903773@0)
put down in black and whites
英语可以至少十种这样的说法，不要太局限，口语简单通俗就可以， write down in a piece of paper, 这是我上班和老外经常说的， -me515(me515); 2017-9-26 (#11072248@0)

Besides, “ink” can be used in informal English as a verb in “to ink a contract” which means “to sign a contract”. e.g. Meisuowei（混混姐）has just inked a big deal ！