singular or plural

Hi,

Sorry but what is the difference?

1.Should it be in a present or future tense?
2.Should it be in a present or a future tense?
3.Should it be in a present and future tense?
4.Should it be in present and future tenses?

5.Can I have a red or blue pen? 
6.Can I have a red or a blue pen?
7.Can I have a red and blue pen?
8.Can I have red and blue pens? 

I think nos. 7 and 8 are clear: no. 7 means one pen in red and blue colors; no 8 means possibly many pens in red and blue colors -- could one pen have both red and blue colors. Come to think of it, no. 5 could mean a pen with both red and blue colors and no. 6 could mean one pen in red and another pen is blue.  

Thank you.

Let me shorten the number of questions. How are they different?

1.Should it be in a present or future tense?
2.Should it be in a present or a future tense?
3. Should it be in a present and future tense?
4. Should it be in a present and a future tense?

I think no. 3 is incorrect, but I am not sure about nos. 1, 2, and 3.
No. 3 could work if the noun is a pen -- assuming a pen has both red and blue colors.

Should it be a red and blue pen?