Conditional highlighting rules are attached to the cell to be highlighted.
The rule
compares the value in the cell to be highlighted
with a fixed value, written into the rule,
or with the value contained in another cell.
If the comparison returns true, the highlight is appplied to the cell to which the the rule is attached.
To do what you want, you will need to attach a CH rule to cell B3, the 'cell to be highlighted.
The rule will need to compare the value in B3 with the value in another cell (for example, K3).
That other cell's content will be dependent on the content of A1, and must be something that can be compared with the content of B3.
Example:
A1 can contain any text value.
B3 can contain any text value.
The CH rule for B3 will be: (if) Text is (cell reference) K3 , red fill
(To insert a cell reference in the rule, click the icon at the right end of the box in the rule editor, then click cell K3)
The formula in K3 is: IF(A1='Hello",B3,"xxzzxx")
xxzzxx can be any text that will never match the content of cell B3.
Regards,
Barry