Pink or purple are both fine, firmness-wise.
Please, please consider getting a grooming table. If it's not something you can buy right now, then try another sturdy table with a towel over it. I would bet that your dog has never in his life been brushed on the floor - but has spent a significant number of hours up on a grooming table. This is just the reality of how we groom show dogs. (It will also be so much easier on your back than sitting on the floor for the hour-plus that it takes to properly brush him out.)
Here's the other thing:
your dog is training you. Think about it: every time he gets uncomfortable around the brush, you give him a treat. If he really flips out when the brush appears, you'll give him more treats! In your most recent post, you're even hunting around for more- and more-delicious treats to give him when he pitches a fit. What a lucky dog!

I know you only mean the best, and that you're using the treats to try and distract him, but in his clever little Sheltie brain, he's learning that bigger fits bring more cookies.
Brushing is not an inherently-traumatic event. Even if (God forbid, and I'm not at all proposing that this is the case) the poor dog was beaten to within an inch of his life with a ball-tippy pinbrush back in the day, you're going to teach him that brushing is OK and normal by having brushing be OK and normal.
Get him up on a table (preferably with an arm, so he can't go far) and let him know, gently but firmly, that brushing is going to be the order of the day and that you, the human, get to brush him until you (and only you) decide that you're done. Be calm and confident. If you need a buddy to help you hold him, that's fine. Cookies are fine too -
if and
when he holds still (go ahead and set the bar low at first on that one... even a few moments of composure on the dog's part are a good start). However, the dog no longer gets to define when brushing ends OR when he gets a cookie. Remember, he's probably spent his whole life with humans who are calm, confident, and fair but firm when it comes to grooming - because grooming is just an everyday reality for these dogs.
But - I'd try a grooming table first, and see if that doesn't make a difference.