It should be fatHa (a).
The last letter is not a vowel. It's Ta-Marbútah (meaning: tied T). It's a letter that is spelled in two ways: H and T. In regular situations when the word is isolate or the word comes at the end of the sentence where there is no need to continue the speech, then this letter sounds (H). Like here. However, if in the middle of the sentence (or beginning of the sentence) where continuation of speech is needed, this last letter must be moved by vowels and thus it becomes (T).
The tied T is untied to make ت when suffixes are attached to it: e.g. قبعتي (qubba3atí: my hat).
Yes, the two colors have different feminine forms. I've discussed this thing in details elsewhere but anyway I'll try to summarize it here:
Some colors in Arabic, which I like to call (basic) have their feminine form in some irregular way to the usual, by adding ـاء (á') - while dropping the initial (A) or Hamza in the beginning:
- Black: أسود (aswad/m) → سوداء (sawdá'/f).
- Red: احمر (aHmar/m) → حمراء (Hamrá'/f).
These are just two examples. I will show the full list below.
The second group is "regular" or maybe I should call it "adjective colors". These colors get their names from a real object, in the same way that Russian sometimes add a suffix to a noun to make an adjective, same thing happens in Arabic: Specific objects get adjective suffix to derive a color name. Examples:
- Coffee bean: بُن (bun) → Brown: بنّي (bunniy/m). [i.e. Color of coffee-bean]
- Rose(s): ورد (ward) → Pink: وردي (wardiy/m). [i.e. Color of the rose].
- Orange: برتقال (burtuqál) → Orange (color): برتقالي (burtuqáliy). [i.e. color of oranges].
So, for the second category, the masculine and feminine forms of the color are simply "adjectives" derived from adding ـي (-iy) for the masculine and ـية (-iyyah) for the feminine. And this derivation of adjectives is not limited to colors in fact; We can derive many adjectives from nouns (e.g. nationalities).
Now, the list of the "basic" colors, as I call them, and they are more or less related to the natural spectrum plus black and white:
- Black: أسود (aswad).
- White: أبيض (abyaDH).
- Red: أحمر (aHmar).
- Yellow: أصفر (acfar).
- Green: أخضر (axDHar).
- Blue: أزرق (azraq].
I hope I didn't miss any color in this list! Anyway, as you may notice, all these colors start with (a-); This is the masculine form. To make the feminine form for these colors, just shape it like زرقاء (zarqá') above; e.g. سوداء - حمراء - خضراء . If you can understand the "root system" upon which Semitic languages usually work, this would be relatively easy to get around. Hope this helps.
I'm not sure really, it could be a phonetic rule that we subconsciously follow. However, it might be related to the fact that in Arabic, words are formed via "roots" and the least number of consonants in a root is 3.
What that means, is that it is more likely that the Shaddah in words like بُن (bun) is original, but because the word is not declined or anything, we simply don't say the shadda at the end. Shadda re-appears when the word is declined in any position in a sentence.
Another example for such words: لُب (lub: core). If this word receives any suffixes for any purpose, the last consonant (b) would be doubled as well: my core لُبّي (lubbí). Also Love حُب (Hub).
What I believe is, in all these words that are seemingly 2-consonant words, there is a hidden Shaddah that doubles that last consonant because the basic structure is based on 3-consonant roots. It is just not pronounced when it is not needed (at the end of the word, like the general rule of dropping vowels at the end of the sentence), but it shows when suffixes are added.
This would be more obvious in verbs because in verbs the rules of derivation for verbal subject/noun and verbal adjective are strongly related to the number of letters in a verb. In verbs, the Shaddah at the end is more prevalent when verbs are discussed, usually.