Why can control be reduced when a large amount of white is added to blue at once to make light blue?

Correct answer: It may become lighter than desired and be difficult to darken accurately

The governing concept is gradual value control in colour mixing. White raises the value of blue, so adding a large quantity at once may make the mixture much lighter than intended. Darkening it accurately afterward requires adding blue in small amounts and can disturb the original hue. Therefore option A is correct; white does not automatically turn blue green, instant drying is not the issue, and a palette does not change the colour by itself.