"The color is ugly."
Translation:La koloro estas malbela.
They can be, theoretically, but you have to be careful because adjectives they don't always mean the same thing when they become verbs.
La viro estas rapida = the man is fast
La viro rapidas = the man is hurrying
The idea there is that in Esperanto, like in English, there's a difference between saying something is "not pretty" and saying it's "ugly". Just because something is not pretty doesn't mean, as the "mal-" prefix indicates, that it is the opposite of pretty- that is, ugly- that is, malbela.
The reason, then, that you can't say "La kolorone estas bela," is that the translation for "The color is ugly" is asked for- not the translation for "The color is not pretty." There is a small but crucial difference in meaning, and therefore a difference in acceptance by the software.