Current best info is this:
But Jeto is waiting for more info. So, this might be subject to change once we have news.
Based on the above, values are assumed to be rounded up, so anything below 1,800 writs would be negligible - meaning 1 writ and 1,799 writs would be the same after conversion. So, there would be no benefit in hoarding 300 writs, over hoarding 100 and converting 200 into deeds.
If that wasn’t the case, and writs were converted by some other metric, then yes, rainbow crystals are better than coloured ones, so you’d want more of them. But if 1,800 writs is needed for a single rainbow crystal, 300 would still just be one crystal.