What is the use of these green and red lines on Snellen/Eye chart?

Question

I read that in eye tests red and green colors are used. For example:

But in Snellen/Eye chart, these are not background colors. These are lines only. What is the purpose of it?

Answer 1

These two lines can be used to detect color blindness in which red cannot be distinguished from green (deuteranopia). The person tested, often a child in that case, must be able to name the colors of the lines. (Reference: Ellen M. Chiocca, Advanced Pediatric Assessment, p 314):

Otherwise, it is also certainly a convenience to allow the test taker to say which line they are comfortable with, e.g., I can read the letters just above the red line but not the ones under it.

Answer 2

Its for a 'Red-Green duochrome test'.

A primary task of the eye care professional is determining the refraction, or optical correction, of a patient. The duochrome red-green test is a standard tool for verification of the final refraction. Traditionally, it is recommended for use both prior to and subsequent to determining the cylindrical or astigmatic component of the refraction.

Red-Green Duochrome testing