Some characters break editor string styling color and everything in the file is red (string color) after first quote ends but script keeps working fine, meaning database accepts those chars. Seems to be case with unicode characters.
begin
dbms_output.put_line(q'#test#');
dbms_output.put_line(q'¤test¤'); -- everything following is red because ¤ quoting
dbms_output.put_line(q'#test#');
end;
I see. I'll log this.
Speaking of which 
In the following statement everything after the word "remark" is marked as comment (green).
SELECT remark FROM DUAL;
This one will not be addressed. It's technically an issue, but highlighting is performed by a simple lexer for speed. There is no complex parsing involved. REMARK is a SQL*Plus comment if it's the first token. REMARK is meaningless within PL/SQL. So even if the lexer is updated to look for REMARK as the first token it'll still be broken within PL/SQL. This is a minor issue, IMO.
No problem. In fact, this is only a minor cosmetic flaw.
In our case, this also only occurs with one table, which unfortunately has a column called "REMARK"