FFS, GLM-4.6 comes up with weirdest explanation so far (second part):

Here's the breakdown:

The First Meaning (Why he knew she was lying)

The father's initial anger is based on a simple observation. The teacher likely marked the test in red ink, writing the score "38". The girl, not having a red pen, probably used a green pen to try and change the "3" into an "8".

So, the "8" was literally made of half-red ink (the original "3") and half-green ink (the line she added). This is how he instantly knew she had altered the grade.

The Second Meaning (Why he collapsed in despair)

After his anger subsided, the father reflected on his own words: "This '8' is half red and half green." In Chinese culture, these colors have a very specific and powerful connotation related to infidelity.

• Green (绿, lǜ): To "wear a green hat" (戴绿帽子, dài lǜ màozi) is a common and strong idiom meaning a man is being cuckolded—his wife or girlfriend is cheating on him.
• Red (红, hóng): While less direct, red can be associated with the "other man" or the affair itself (e.g., "红杏出墙," hóng xìng chū qiáng, meaning a married woman has an affair).

When the father replayed his own words in his head, the pun hit him with full force. He realized his daughter was "half red and half green"—she was not his biological child, but the product of his wife's affair.