When you use the colors in this manner they become declined as adjectives. The adjective in german when used this way declines into the same gender and number as the noun they are describing. In your examples Auto is neutral singular, the adjectives in neutral singular (nominative) end in -es,hence gelb-es Auto/ rot-es Auto. It would do the same if instead of saying a color you said simply that it was a big (groß) car: groß-es Auto. Blumen are plural so the adjective ends in -en: orang-en Blumen /schön-en Blumen. Lila however is an exception, this color doesn't decline at all and that's why it wasn't corrected.

Here's what the book 'German Grammar', by Dr. Paul G. Grades, says about declensions: If an adjective is preceded by words like der (the), dieser (this), jeder (each), welcher (which), solcher (such) or all(er) (all), it takes "weak" endings: If an adjective is not preceded by any of the words mentioned above, it takes "strong" endings. Adjectives preceded by indefinite pronouns or numerals take strong endings. EXCEPTION: when using keine and alle, the adjective following these indefinite expressions take weak endings. If an adjective is preceded by words like ein (a, one), mein (my, mine), sein (his), ihr (her) kein (no, none), unser (our) or euer (your), it takes "mixed" endings. There are three neat tables in the book for each type of ending. You might want to make a some tables yourself to refer to. Weak endings: Nominative Singular -e Accusative Masculine Singular -en Accusative Feminine or Neuter Singular -e Dative (all) -en Genetive (all) -en All plurals -en Strong endings: Nominative Masculine Singular -er Nominative or Accusative Feminine Singular -e Nominative or Accusative Neuter Singular -es Nominative or Singular Plural -e Dative Masculine Singular -em Dative or Genetive Feminine Singular -er Dative Neuter Singular -em Dative Plural -en Genetive Masculine Singular -en Genetive Neuter Singular -en Genetive Plural -er Mixed Endings: Nominative and Accusative singular are the same as their strong endings. All plurals, as well as everything in dative and genitive, take their weak endings.

To understand the endings you must study the 4 grammatical cases - nominative, accusative, genitive and dative

When it comes to languages, I just feel doing is the best way. After all, a child speaks their own language correctly long before they ever attend a grammar class at school.

I definitely learn faster by doing, rather than by studying. I have dyslexia (only mildly obviously) but I am a very visual, tactile learner as a result. I have never opened a Chinese grammar book in the whole 6 years I have been studying. I remember things much more easily by reading everyday materials, watching videos, speaking with people, asking questions etc etc. By the time I got to uni to study it (4 years in) I was already reading and understanding newspapers, short stories and was able to hold quite in depth, complex conversations with teachers. I hated studying it at university. It was so formal. The textbooks were confusing, and explanations about even the simplest structures seemed so much more complicated than they needed to be. The teachers, knowing I spoke Chinese already would often ask me to explain grammar to classmates, esp the particle 'le'. Friends would ask "Why does le go here?" and my reply was always " I don't know, I just know it does. It feels right there". German grammar is much more complicated than Chinese and I am realizing I need to worry about grammar more than with Chinese, but I'm trying to pick it up as I go along as it were. I'm making notes about what I notice in my own words and so on.