Learn English – Which tense should I use when describing the following facts

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.

Consider a situation where you called on Tom at his home. The following day, you can say to a mutual acquaintance

Tom was at home yesterday

In this context, it is neutral about where Tom is now. He could be still at home: he could be somewhere else.

Now suppose that, when you were at Tom's house, you asked him where he came from, and he replied

I am from London.

When you tell your mutual acquaintance, you say to him

Tom said he was from London. - reported speech - backshifted
Tom is from London - direct statement

You can use is because Tom is definitely still from London.

Similarly, when you interview your client, you might get these answers:

My name is Tom
I am 34 years old
I work at XYZ bank
My background is in IT
I work in accounts

When you are writing up your report a few months later, you would be obliged to say

His name is Tom - still true
He was 34 years old - at the time of the interview
He worked at XYZ bank - at the time of the interview
His background is in IT - still true
He worked in accounts - at the time of the interview

I am not suggesting that you should always switch between present and past like this: it is just an example that shows how you can express these ideas precisely if required.

If it's not necessary to be precise and you have multiple items of information, it's probably best to stick to whichever is the most common of present or past: the context will usually be enough to avoid misunderstanding.

If precision is important, it depends exactly what you mean by "from the IT field". If it means "His training is in IT", then it's still true, so you should use is. If it means "He is currently working in IT", you should use was.