Because of keyword <script> UltraEdit detects that this section is a javascript section. So the javascript language definitions for syntax highlighting get active for this section.
In default Wordfile.txt in the UltraEdit program directory at /L9"JavaScript" in the color group /C3"Methods and Properties" the lowercase word "index" is listed. Javascript is case-sensitive.
You have also activated at
Advanced - Configuration - Editor Display - Syntax Highlighting the option
Auto Correct Keywords. So UltraEdit detects this wrongly written keyword and corrects it to lowercase.
You have now 4 possibilities:
1) Turn off "Auto Correct Keywords" which is a global setting for all languages.
2) Remove "index" from the C3 section of javascript.
3) Add also "Index" to any color group of Javascript. For example to not existing color group /C6"MyVariables".
Note: Because javascript language is case-sensitive you have to add "Index" to the line with the words starting with uppercase I and not with lowercase i !
4) Simply do not use keywords of a language as variable or function name.
See also IDM's power tip
Autocorrect keywords in UltraEdit/UEStudio.