You may want to contact the LEGO customer service about it and see if they can provide a replacement brick. Assuming your train is 10874 or 10875, they should still have it in stock and you can probably just order a new one via a replacement part order.
On the other hand, since the recognition appear to be indeed color-based this might not help at all.
I don't know what the equivalent in your country is, but I've had a lot of luck with Craigslist in the USA. When my daughter first got interested, I found a table with a TON of bricks for about $100, and I regularly see bulk DUPLO lots in the $30-80 range.
There are always "starter packs" with more of the simplest Duplo bricks available, at the time of the writing of this answer those are the sets #10848 (My First Bricks), #10572 (lego duplo all in one box of fun), and #10887 (Creative Fun).