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.
Using this as a reference, these sound OK, with the exception of "have to" but only because repeating "have" twice like that sounds a bit jarring.
I ought to have finished the homework by the end of the week.
I have to have finished the homework by the end of the week.
I shall have finished the homework by the end of the week.
I have heard this before, it sounds a bit awkward but makes sense.
I had better (typically "I'd better") have finished the homework by the end of the week.
I have also heard this before, it sounds quite awkward but still makes a bit of sense.
I have got to (typically "I've got to") have finished the homework by the end of the week.
This ...
I must have finished the homework by the end of the week.
I must not have finished the homework by the end of the week.
sounds like you are just discovering you have or have not done the homework, rather than using "must" to communicate a requirement or obligation.
However, usually all these sentences are arranged like this, which sounds much more natural to me.
I ought to have the homework finished by the end of the week
And all the modals "work", including the ones not mentioned above, if you keep the sentence like that:
I can(n't)/could(n't)/must(n't)/might (not)/may (not) have the homework finished by the end of the week.
(Even though "I must not have the homework finished by the end of the week" - meaning I am required to not have my homework finished by the end of the week - is a peculiar situation to be in - that would be the right way to say it.)
Best Answer
Both of these woulds represent future constructions cast into past form to agree with the 'hypothetical' past form in their respective if clauses. They might with perfect propriety be expressed using present forms:
To which you might revert is a present form with future reference, again cast into the past form to express hypotheticality. Its Event time is its main clause’s Reference time, in a few years. Once more, all of these could be expressed with present forms:
Jane, however, is careful to keep her moralising in the hypothetical mode, whether because she is reluctant to instruct too directly an older and far more sophisticated man, who is moreover her employer and social superior, or because she is “sensible that the character of my interlocutor was beyond my penetration; at least, beyond its present reach; and feeling the uncertainty, the vague sense of insecurity, which accompanies a conviction of ignorance.”