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.
I think either form would be acceptable, but something else is bothering me about the sentence. I would move the word “directly” to the end of the sentence:
The only way to communicate with me is to send me an email directly.
The only way to communicate with me is sending me an email directly.
Of these cases, I would prefer to hear the first option. It sounds more natural to me.
You can make the second sound more natural by adding “by”:
The only way to communicate with me is by sending me an email directly
Adding “by” has a lot of impact on how I read the sentence, and it suddenly feels very natural. Not sure why, though. I would say that when using this form, always use “by”.
Best Answer
Marrying the two constructions BE to VERB and BE VERBing in the construction BE to be VERBing is syntactically acceptable.
However, it is very formal, and in many cases of little value.
BE to VERB itself is largely confined to very formal registers. Moreover, it is rather vague: I cannot think of a situation when it should be preferred to a more precise expression such as BE expected to VERB or BE required to VERB or BE scheduled to VERB or something else of that sort.
With 'atelic' verbs like study, run, sleep (verbs which don't have a 'built-in' endpoint or change of state), the progressive infinitive to BE VERBing adds nothing to the sense of the plain infinitive. It may be of some value in imposing an 'atelic' sense on a telic or ambivalent infinitive, such as to present, to make, to write.
It's good that you understand how this construction may be used; but I advise you to avoid it in your own work if it's not necessary. Your first example, for instance, appears to mean something which would be much more natural like this:
Keep in mind, too, that the progressive itself may bear the sense of something expected or planned. If that's the sense you intend you may omit the BE to (or paraphrase) altogether:
Your second example is a little different. Here I suspect that what you mean is:
Studying there is not a progressive but an adjunctive participle. In some circumstances, however, a progressive may have emphatic or contrastive force: