Hey
if the litmus paper is dry obviously there would be no effect.
But this question is quite tricky because though CO is neutral it will turn the wet blue litmus red. It is because
CO + H2O -----> CO2 + H2

The Carbon Dioxide so formed is an acidic oxide which will further react with water to give carbonic acid
CO2 + H2O ----> H2CO3
H2CO3 i.e. the carbonic acid will give H+(Aaq) ions which will turn blue litmus (wet) to red
That's all. Hope it helps

Will carbon monoxide change the colour of blue litmus?

No, carbon monoxide does not change the color of blue litmus paper. Blue litmus paper is used to test for the presence of an acid. When an acid is present, the blue litmus paper turns red. However, carbon monoxide is not an acid and therefore does not have the ability to change the color of blue litmus paper.

Explanation:
Carbon monoxide is a colorless and odorless gas that is produced from the incomplete combustion of carbon-containing fuels, such as gasoline, coal, and wood. It is highly toxic and can be life-threatening when inhaled in high concentrations.

Blue litmus paper:
Blue litmus paper is a pH indicator that is commonly used to determine whether a substance is acidic or basic. It is made by impregnating filter paper with a blue dye derived from lichens. When this paper comes into contact with an acidic solution, the blue litmus paper turns red. This color change occurs due to a change in the pH of the solution.

Carbon monoxide and blue litmus paper:
Carbon monoxide is a neutral compound, meaning it is neither acidic nor basic. As a result, it does not cause a change in the pH of a solution when it comes into contact with blue litmus paper. Therefore, blue litmus paper will remain blue when exposed to carbon monoxide.

Conclusion:
In summary, carbon monoxide does not change the color of blue litmus paper. Blue litmus paper is used to detect the presence of an acid, and carbon monoxide is a neutral compound that does not exhibit acidic properties.