sed -r 's/([A-Z]+):(.*)/1:U2/;s/([A-Z][a-z]+):([a-z])/1:U2L/' file

$ cat case.awk
/^[[:lower:]]+$/ { print "lower"; next }
/^[[:upper:]]+$/ { print "upper"; next }
/^[[:upper:]][[:lower:]]+$/ { print "capitalized"; next }
/^[[:alpha:]]+$/ { print "mixed case"; next }
{ print "non alphabetic" }

Jims-MacBook-Air so $ echo chihuahua | awk -f case.awk
lower

Jims-MacBook-Air so $ echo WOLFHOUND | awk -f case.awk
upper

Jims-MacBook-Air so $ echo London | awk -f case.awk
capitalized

Jims-MacBook-Air so $ echo LaTeX | awk -f case.awk
mixed case

Jims-MacBook-Air so $ echo "Jaws 2" | awk -f case.awk
non alphabetic

BEGIN { OFS = FS = ":" }
$1 ~ /^[[:lower:]]+$/ { print $1, tolower($2); next }
$1 ~ /^[[:upper:]]+$/ { print $1, toupper($2); next }
$1 ~ /^[[:upper:]][[:lower:]]+$/ { print $1, toupper(substr($2,1,1)) tolower(substr($2,2)); next }
$1 ~ /^[[:alpha:]]+$/ { print $1, $2; next }
{ print $1, $2 }

$ echo BEHAVIOUR:behavior | awk -f case.awk
BEHAVIOUR:BEHAVIOR

$ echo Behaviour:behavior | awk -f case.awk
Behaviour:Behavior

$ echo behaviour:behavior | awk -f case.awk
behaviour:behavior

awk -F ':' '
   {
   # read Pattern to reproduce
   Pat = $1
   printf("%s:", Pat)

   # generic
   if ( $1 ~ /^[:upper:]*$/) { print toupper( $2); next}
   if ( $1 ~ /^[:lower:]*$/) { print tolower( $2); next}

   # Specific
   gsub( /[^[:upper:][:lower:]]/, "~:", Pat)
   gsub( /[[:upper:]]/, "U:", Pat)
   gsub( /[[:lower:]]/, "l:", Pat)

   LengPat = split( Pat, aDir, /:/)

   # print with the correponsing pattern
   LenSec = length( $2)

   for( i = 1; i <= LenSec; i++ ) {
      ThisChar = substr( $2, i, 1)

      Dir = aDir[ (( i - 1) % LengPat + 1)]
      if ( Dir == "U" ) printf( "%s", toupper( ThisChar))
       else if ( Dir == "l" ) printf( "%s", tolower( ThisChar))
        else printf( "%s", ThisChar)
      }
    printf( "n")
    }' YourFile

sed -r '/^([^:]*):1$/Is//1:1/' file