Prepaid sims are purple/green. Do you have the orange/pink one?

As for the sim card, does your current phone use the same size as the new one? If so you can just take the sim out of one and put it in the other and save that sim card you have as a spare.

Dunno the colour, only have the sim, but I was assured that it was for my prepaid. 

Why can I not activate it?

Hoe did you get the SIM if you can't even see the colour? 

Uh see, I HAD the card it came in but that was years ago and I only have the sim now, which doesn’t have the colour coding on it.

It sounds like if the sim was taken out of the card, the sim likely would have been activated already and wouldn't be able to be reactived after 90 days without a baseplan 

I received a sim card as a gift, it was contained within one of those coloured cards. I REMOVED the sim card from the coloured card.

Now, years later I want to use the sim card because my new phone needs a mini sim card. The sim card for my old phone doesn’t fit.

When I go to activateprepaid.koodomobile.com and try too activate it, it says “SIM card number is not available to be activated”!

When I go to https://prepaidselfserve.koodomobile.com/Overview/products-and-services/change-sim-card/ and try too activate it, it says “Invalid SIM number; please check the 19-digit number entered”, which I have!

WHY CAN I NOT ACTIVATE MY SIM CARD!?!?!?

The most likely answer is that it's not a prepaid sim card, or its a card that's been previously activated. But since you popped it out of the card years ago it will be tough to confirm 

Is there no way of searching the number in the koodo database and finding out?

You can request a callback via koodo assist and a rep can confirm the sim card number. Let us know what they tell you. 

https://www.koodomobile.com/en/help?autobots=rollout

Thanks! You kept me out of troubles today.

Apparently, not so many know the “color code”. 

Not even  some of Telus/Koodo employees.