It is all about the use of adjectives.Adjectives in Danish have 3 forms ie The grundform used for n-words The t-form used for t-words and The e-form used for flertals,after den and det,after possessive pronominer( min,din,hans vores ect),and after genitiv( hans, hendes ect). The sentence above does not tell us what is red and white but the ts at the end of red and white makes us to know it is a t-word. If it was an n-word,it will be det er rød og hvid. If it was plural then it will be røde og hvide. Hope this helps abit.
Yes. If you have a definite article before an adjective, you use the plural form of that adjective, usually by adding "e", like smukke, røde, store. But not all adjectives have a plural form that ends with "e": den blå bil, det lilla hus, det små barn. You also use the plural form when there is a possessive pronoun before the adjective: Min store bil, etc. Finally, if you have the indefinite articles "et" or "en", you use the singular form of that adjective (but mind the gender of the noun!): en ny bil, et stort hus, et gammelt dyr, en smuk kvinde.