Learn Swedish: Det lilla huset är rött.

This is a classic Swedish rule called double definiteness. When a definite noun (like huset, meaning the house) has an adjective in front of it, you must also add a freestanding article before the adjective. Since hus is an ett-word, we use Det. So literally, the structure feels like saying "The little the-house."

Why do we use lilla instead of litet or liten here?

Lilla is the definite form of the adjective liten (small/little). Whenever you use the double definite pattern (Article + Adjective + Definite Noun), the adjective must take its definite form. Usually, this just means adding an -a to the end of the word, but liten is irregular and changes to lilla.

Why is the color spelled rött with a double 't' instead of just rödt?

The adjective röd (red) has to match the ett-word hus. Normally, you just add a -t to an adjective to match an ett-word. However, because röd ends in a -d following a long vowel, the -d is dropped and -tt is added instead. This spelling change also tells you to use a short vowel sound when you say it!