Have been explained in the Ludicrous version.

6th example: RBRGBRBGGRRRBGBBBGG

If going by your logic of start + end = result: R + G => B

But if we iterate manually:

R B R G B R B G G R R R B G B B B G G
 G G B R G G R G B R R G R R B B R G
  G R G B G B B R G R B B R G B G B
   B B R R R B G B B G B G B R R R
    B G R R G R R B R R R R G R R
     R B R B B R G G R R R B B R
      G G G B G B G B R R G B G
       G G R R R R R G R B R R
        G B R R R R B B G G R
         R G R R R G B R G B
          B B R R B R G B R
           B G R G G B R G
            R B B G R G B
             G B R B B R
              R G G B G
               B G R R
                R B R
                 G G
                  G

So... no. It's not B. It's G. The notation of start + end = result is not always correct.

Not a kata issue. Resolved.