I have the same problem for the latest SciTE version "3.7.3 Feb 16 2017 21:41:17".
Following the instructions found at the bottom of this page: https://github.com/downpoured/scite-files/blob/master/files/api_files.md, I successfully enabled calltip and autocomplete for user udfs.
However, syntax highlight (i.e., highlight for user udf function names) does not work despite having used SciTE Config to change its color. The text for user udf function names remains black. In contast, I am able to see different colors for standard udfs.
The following describes my setup:
My *.api and *.properties files are located in the same directory as SciTE.exe.
(In addition, I do have a SciTE folder in %LocalAppData%\AutoIt v3)
I have tried pasting the contents of my *.properties file in every seemingly related properties file I found, e.g., SciTEUser.properties.
(The content of my *.properties file consists of just: au3.keywords.user.udfs="function_names \")
Finally, I found that attaching my function names to existing "au3.keywords.udfs" definitions will not confuse SciTE into treating my functions as standard udfs.
Having decided that most wikis do not provide a substantially different install method, I'm not sure what else to try now.
If someone suspects a mistake anywhere, please let me know. I'm new to autoit, so feel free to assume mycomplete noobness