can anyone explain why venstre and rød are declined differently even when describing the same object?
What do you mean? It could be because sko is used for both the singular and the plural
Well in this sentence it is clearly singular as indicated by the word din instead of dine.
Of course, I just misread what you said the first time, but venstre is the only way that word can be declined, the normal and the t-form are both still venstre. But normally after the definite article one would use the "e-form" instead of the common or t-form, for example Den store sko er rød, this also happens after the possessive pronouns (Min/Din/Sørens store sko)
"Den venstre sko er stor og rød". "Den venstre, store sko er rød". "Den venstre, store, røde sko".