Here is the original item:
With her heavy dictionary,
(A) Donna
thumps the top of her ancient computer
monitor. (B)
Whenever the color tint flickers
(C) from
normal to sickly yellow.
You wanted to do this:
With her heavy dictionary, Donna thumps the top of her ancient computer
monitor. Whenever the color tint flickers. From
normal to sickly yellow.
Adding a period after flickers will make from normal to sickly yellow a fragment.
You might want to review the rules.