The step by step guide by the OP is very clear. There must be something you may have done wrong without realising.
Best thing to do is double check you have entered everything properly. for example...

The OP wrote

db manipulation
open the table "products" in your zencart-database and insert new rows at the end of the table called "products_guarantee" and "products_color"
ALTER TABLE `zencart_products` ADD `products_guarantee` INT NOT NULL , ADD `products_color` VARCHAR( 32 ) NOT NULL;
did you change the table names to suit your database, ie: ALTER TABLE `zen_products`............

Did you do as I said in my previous post.. adding the comma ","

In collect_info.php and preview_info.php, when you add the lines p.products_guarantee, p.products_color (or what ever you called them for your database) you need to make sure to add a comma at the end of the last line before you add the above lines.

I know, very simple things, but easy to miss too.

Im not great at explaining things, but hope it helps

DB