I just did a few minutes of testing on this. I'm almost positive that this will vary by stage and the weight of the character, so for clarification I was on Final Destination vs Lucario. Keep in mind that I only did this stage versus this character. I did not do any extensive testing, but I did enough to know that the following is true:
For the knockback to have any effect at all, you have to hit the person directly with the last pikmin on your tether. So you have to be at ~70 degree angle from them to hit them, because of Olimar's little short hop up before the tether shoots out.
I found that of the 5 colors of pikmin, all but yellow had the same spike effect downwards. The yellows seemed to knock the opponent to the side away from the stage(ex, right ledge on the stage, the yellow pikmin sent it to the right) but not far enough to matter very much at 0, 30, or 50%.
Of the four that spiked downwards, purple seemed to be very slightly more effective than the other three. I didn't notice any difference between red, white, and blue (no pun intended) surprisingly--I was expecting the white to be weaker--but they didn't seem to be.
Basically the same results for purple, white, red, and blue-
At 0% damage, lucario was able to recover fairly easily, although it did knock him down off the screen, he still recovered.
At 30%, lucario was BARELY able to recover, mid-air jump + bup barely got him to the ledge.
At 50%, lucario was stunned too badly and was dead before he could do anything.
That's all the testing I did, and am probably going to do, at least for tonight.
From what I can tell, this would be a good weapon to use if you get gimped and can't recover(no purples, miss on the throw, etc)