How to NESTING IF STATEMENTS IN EXCEL when the last iteration of character ":" is unknown number of CHARACTERS from the right

Heres my code: =if(left(right($C181,5),1)=":",right($C181,4),if(left(right($C181,6),1)=":",(right($C181,5),if(left(right($C181,7),1)=":",right($C181,6)))))

Parse error

SAMPLE DATA -- ALMONDS:N1202:8103

ALMONDS:N1202:9103L

ALMONDS:N1202:N1202M

ALMONDS:N1202:N1202P

I NEED THE STRING AFTER THE LAST ":" IN EACH INSTANCE WITH THE NUMBER OF CHARACTERS IN THAT STRING UNDETERMINED BUT BETWEEN 4-8 CHARACTERS.