Hola Carole,

In "Me gusta la de rojo." we are using "la" because we are referring to something feminine, for example "a girl". You could also say "Me gusta el de rojo." if you were referring to a guy wearing red. The same applies to "Los de mañana serán mas frescos"; here the speaker is referring to something masculine plural, e.g los huevos (eggs), los pasteles (the cakes). But again, if you say "Las de mañana serán mas frescas", then the speaker may be talking about something feminine plural, e.g las manzanas (apples). 

The box in the lesson where you can see these examples is a box simply explaining that you can find adjectives, nouns or adverbs after this structure and we didn't explain in these cases whether the references were masculine or feminine, as that had already been explained before. 

I hope this helps.

Saludos