Technically, the tense should match, so since we're using the perfect tense (conditional perfect in fact), you should use was.
But both could have ... is and could have ... was are acceptable in this case.
Why? Well, it's because your clause about the paprika being similar to the bell pepper might be true for a long time, and might continue to be true even in the present, so it might be okay to use is.
Maybe not. Maybe the paprika is rotten by now, or already eaten. Then is would make no sense. But a situation continuing to the present would justify use of the present tense is.
Consider:
"I could have gone shopping, because the supermarket was near."
"I could have gone shopping, because the supermarket is near."
If the supermarket has not suddenly moved, it probably still is near – it is now, and it was then – so either one is acceptable.
But some things do not last so long:
Correct: "I could have stayed longer, because it was early in the morning."
Incorrect: "I could have stayed longer, because it is early in the morning."
Unless you are describing something in the very recent past (minutes or hours ago), the fact that it is now early probably has nothing to do with the situation in the first half of that sentence, so mixing the past and present tense in this last example doesn't really work.
According to the British Council:
It isn’t always necessary to change the tense. If something is still true now ... we can use the present simple in the reported sentence.
Note the word necessary. In many contexts, it is still permissible to backshift the tense, even if something is still true now.
She said, "When do the banks close?"
1) She asked me when the banks closed.
2) She asked me when the banks close.
The first version (with a backshift) is definitely the best choice if we are talking about a conversation that took place many years ago, as bank closing times may have changed since then. The second version is better if you are talking about a conversation that took place last week, but you could also use the first version.
He said to me, "What does this word mean?"
1) He asked me what this word meant
2) He asked me what this word means
It is very unlikely that the word has changed its meaning since the conversation took place, so either version is acceptable. I think that I would go for the first version (with a backshift), though I cannot explain my preference in any way.
Note that, unless you want to specify when you learnt about something "I have known about it for some time", you generally use know in the present tense: "I know that..."
Best Answer
The second example is straightforward, it's the passivisation of "company A built this building". Very straightforward. No question of what it means.
The first example is more complex. It can be parsed in multiple ways. It could be the passivisation of the present tense:
That's not actually a structure you'll come across that often. If the building is going on now, it would be "is building", and if the building were finished it would be "built". If there were several buildings, or a category of building, and there's a long-running scheme of building them all, you could have:
But that that's not what we're dealing with. That sort of present tense in the singular, with a specific building, will mostly be seen on a timeline, referring to either the past or the future, where people are listing events with when they happened or are expected to happen.
However, that's not the only way to parse it. I can find little formal support for this, with dictionaries not having such things as examples that I've yet been able to find, but I know from experience that it is reasonably common usage to use "built by X" as a participle phrase, making it function as an adjectival phrase. There, built is the past participle, and is built up into a participle phrase with the prepositional phrase "by X", and the whole phrase becomes an adjective.
I don't know what any formal grammar would make of that, but it is a usage I've come across - and even used - several times in my life, and that's the best way I can make sense of it. Taken that way, "built by company A" is effectively an adjective, in your first example, and the use of is is completely natural.