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#Java 8, 228 227 bytes

s->{int l=s.length();return l>6?s:l>5?s+s.charAt(5):l>4?s.replaceAll("(.)(.)(.)(.)$","$1$4$2$4$3$4"):l>3?s.replaceAll("([^#])","$1$1"):l>2?s.replaceAll("([^#]{2})","$1$1$1"):l>1?s.replaceAll("([^#])","$1$1$1$1$1$1"):"#000000";}

I have the feeling some parts can be golfed, especially the regexes. Regexes aren't really my expertise..

PS: When I change "$1$1", "$1$1$1" and "$1$1$1$1$1$1" to String t="$1$1";, x, x+"$1" and x+x+x the byte-count stays the same.

Explanation:

Try it here.

s->{                                       // Method with String parameter and String return-type
  int l=s.length();                        //  Length of the String
  return l>6?                              //  If the length is 7:
    s                                      //   Return the input-String
   :l>5?                                   //  Else-if the length is 6:
    s+s.charAt(5)                          //   Return the input-String with the last character repeated
   :l>4?                                   //  Else-if the length is 5:
    s.replaceAll("(.)(.)(.)(.)$","$1$4$2$4$3$4")
                                           //   Use a regex with capture group to return the correct #ADBDCD pattern
   :l>3?                                   //  Else-if the length is 4:
    s.replaceAll("([^#])","$1$1")          //   Repeat every character, except the starting #
   :l>2?                                   //  Else-if the length is 3:
    s.replaceAll("([^#]{2})","$1$1$1")     //   Repeat every two characters three times, except the starting #
   :l>1?                                   //  Else-if the length is 2:
    s.replaceAll("([^#])","$1$1$1$1$1$1")  //   Repeat the character 6 times, excluding the starting #
   :                                       //  Else:
    "#000000";                             //   Simply return the literal String #000000
}                                          // End of method