Because you cannot color a checkbox output value (as a checkbox; it outputs as a value of 0 or 1) and because you are probably looking for something is less-complicated to implement; you would be better using a text-output value.
Formula-Rich-Text field:If( [Week2 Date] - [Week1 Date] > Days(7), "<div style=\"color:red;\">> 7 Days<div>","")
This displays in red text
> 7 Days and displays no value when the criteria is false.
To display colored-check marks you would need to download and store some custom images to be used instead of text as an output. Quick Base does not have a red or black check image in their Icons
list which would have been ideal. If any of the other icons Quick Base has, work for your needs; then your fomula could be:
If( [Week2 Date] - [Week1 Date] > Days(7),
"<div><img src=\"
https://images.quickbase.com/si/24/221-point_red.png \" alt=\"\" title=\"> 7 Days\" width=\"16\" height=\"16\" /></div>",
"<div><img src=\"
https://images.quickbase.com/si/24/222-point_green.png \" alt=\"\" title=\"< 7 Days\" width=\"16\" height=\"16\" /></div>")
If you wanted to see the number of days as your output then you could do this:
var text daysBetween=ToText(ToDays([Week2 Date] - [Week1 Date]));
If([Week2 Date] - [Week1 Date] > Days(7),
"<div style=\"color:red;\">"&$daysBetween&"<div>",
"<div style=\"color:black;\">"&$daysBetween&"<div>")
