In a dedicated Darling in the FranXX mode, the best starting guesses are the characters that give you the clearest visual and release-year anchors inside the franchise. Because the series is already fixed, your opener should help separate the remaining cast by hair, eyes, and era as quickly as possible.
Darling in the FranXX mode draws on 5 characters and every one of them is listed for 2018, so the year tile is dead weight on this board and the solve has to come from visual clues and process of elimination.
The pool splits 3 male to 2 female, so a gender match usually removes about 3 of the 5 candidates in one go.
Pink hair is the tightest visual filter here, belonging to only 1 of the 5, while white hair is the broadest group with 2.
On eyes, dark is the rarest tag (1 character) and blue is the most common (3).
Hiro and Mitsuru agree on 4 of the five clue traits, and that pair is what decides most failed runs in this mode.
If you only memorise one character first, make it Zero Two — it carries the most distinctive clue profile in the Darling in the FranXX pool.
Your opener should probe the widest axis left in the pool, because the series clue is already spent. Darling in the FranXX mode has 5 characters, all listed for 2018, where pink hair appears on only 1 of them, while white covers 2.
Avoid opening on pink hair: it belongs to only 1 character, so a miss there teaches you almost nothing. A better second guess changes era or eye colour, and Zero Two is a safe anchor for that job.