I first thought of Nanase having to transform into a gecko to climb the walls, but wasn't sure what the actual puzzle would be. I just wanted a lizard form and wall climbing.

I later thought of there being a pattern, meaning Nanase would need help just to position her feet properly.

At this point, I dreaded having to draw the pattern.

Thinking that help with positioning her feet wasn't enough, however, I tried to think how to make it worse, and then considered the color blindness thing.

It was only after briefly dreading drawing the pattern again that I realized that the comic's in greyscale, and it actually made sense not to draw it at all given the puzzle.

I was thrilled.

Of geckos, color blindness, and the color red

As added to the previous commentary, the color blindness bit has nothing to do with her being in a gecko form, and is not meant to be a gecko "fun fact" (it would be incorrect). The gecko form is specifically for sticking to walls. The color blindness is a separate deal to make her have to rely on Sarah for help.

In any case, it is possible to select two different colors that would be almost, if not entirely, indistinguishable from another if colorblind. The exact requirements would depend on the exact type of colorblindness, but it's possible.

Below is a simplified example (color blindness generally wouldn't result in literal grayscale). The red and green in the first row are easy to tell apart even if desaturated, but the red and green in the second row become almost identical.

As for why red and green in the comic, it's because green is complementary to red, and I wanted Nanase to make the point about the color red and instinct.

Due to blood and how people blush red when angry and such, primates have a lot of instincts associated with the color, and it does make sense that with that comes certain associations and color choices for signs and general communication.

Granted, it's not a guarantee of color choice, but it helps explain their thinking.