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.
In your example sentence, you can use "go" if you have not yet gone to the party.
You could use "went" either before going, or after going.
As a side note, I would probably prefer to say "I just wanted to check whether..." rather than "I just wanted to check if..." That's a personal preference. I think many people would agree with me, but I have definitely heard it both ways.
Best Answer
The sentence is in passive form. and happened in the past ( past passive tense)
When you say "I was informed" it is in past passive tense and means somebody informed you of something. But when you say "I informed" it is in simple past tense and means you yourself informed others of something.