Learn Swedish: Jag tänkte köpa en brudbukett med pastellfärger. - I was thinking of buying a bridal bouquet with pastel colors.

This is a common way to be polite in Swedish. Just like in English, using the past tense tänkte makes the statement softer and less demanding than saying jag tänker köpa (I intend to buy) or jag vill köpa (I want to buy). It expresses a present wish or plan, but leaves polite room for the shop assistant to step in and offer help.

What is the difference between starting this sentence with Jag tänkte köpa versus Jag skulle vilja köpa?

Both are polite ways to talk to staff. Jag skulle vilja köpa translates to 'I would like to buy' and is a direct, polite request. Jag tänkte köpa ('I was thinking of buying') states your intention instead of directly asking for the item. It is a very relaxed and natural way to open a transaction in a Swedish store.

Why is there no att (to) between tänkte and köpa?

When the verb tänka is used to mean 'intend to' or 'plan to', it acts like an auxiliary (helping) verb and takes a bare infinitive. You simply say tänkte köpa, never tänkte att köpa. If you wanted to say you were merely pondering the idea, you would add a preposition: tänkte på att köpa (was thinking about buying), but that loses the pragmatic politeness used here.

The sentence uses med pastellfärger (with pastel colors). Could I use the preposition i (in) instead?