Actually "to" and "too" are not really homophones. It is bad enunciation that makes them so, as "too" should be slightly longer when spoken. Correct enunciation of the word "colour" will include the "u", although it may be difficult to detect.
The word "donut" is an unfortunate Americanism, and such diminution makes it harder to realise the origin and meaning of words. A "doughnut" is a small cake made by frying a lump of sweetened dough.
While that may be true in Great Britian I can assure you that here in the states to, too, and two all sound identical when used in conversation.
Regarding the above mentioned words, two of course is the pronunciation of the letter 2. Too is used to indicate an excess such as 'I had too much to eat last night' or 'I was driving too fast and almost crashed!'
To is a bit more difficult to easily outline so I'll go to the dictionary for some help with its definition.
to
to͞o,tə/
preposition
preposition: to
1.
expressing motion in the direction of (a particular location).
"walking down to the mall"
expressing location, typically in relation to a specified point of reference.
"forty miles to the south of the site"
expressing a point reached at the end of a range or after a period of time.
"a drop in profits from $105 million to around $75 million"
(in telling the time) before (the hour specified).
"it's five to ten"
approaching or reaching (a particular condition).
"Christopher's expression changed from amazement to joy"
expressing the result of a process or action.
"smashed to smithereens"
2.
identifying the person or thing affected.
"you were terribly unkind to her"
identifying the recipient or intended recipient of something.
"he wrote a heart-rending letter to the parents"
3.
identifying a particular relationship between one person and another.
"he is married to Jan's cousin"
in various phrases indicating how something is related to something else (often followed by a noun without a determiner).
"made to order"
indicating a rate of return on something, e.g., the distance traveled in exchange for fuel used, or an exchange rate that can be obtained in one currency for another.
"it only does ten miles to the gallon"
Mathematics
indicating the power (exponent) to which a number is raised.
"ten to the minus thirty-three"
4.
indicating that two things are attached.
"he had left his bike chained to a fence"
5.
concerning or likely to concern (something, especially something abstract).
"a threat to world peace"
6.
governing a phrase expressing someone's reaction to something.
"to her astonishment, he smiled"
7.
used to introduce the second element in a comparison.
"it's nothing to what it once was"
infinitive marker
to: to
1.
used with the base form of a verb to indicate that the verb is in the infinitive, in particular.
expressing purpose or intention.
"I set out to buy food"
expressing an outcome, result, or consequence.
"he was left to die"
expressing a cause.
"I'm sorry to hear that"
indicating a desired or advisable action.
"I'd love to go to France this summer"
indicating a proposition that is known, believed, or reported about a specified person or thing.
"a house that people believed to be haunted"
forming a future tense with reference to the immediate future.
"he was about to sing"
after a noun, indicating its function or purpose.
"a chair to sit on"
after a phrase containing an ordinal number.
"the first person to arrive"
2.
used without a verb following when the missing verb is clearly understood.
"he asked her to come but she said she didn't want to" Regarding the use of 'Americanisms' as I'm an American that is only to be expected. Our country is famous for integrating words from the entire world and Americanizing them. With our huge melting pot of millions of people from every conceivable part of the planet living here and becoming part of the American dream it's only natural that we absorb, digest, and I guess in your opinion, mutilate many words. So be it, we're basically the Borg of the written and spoken word. Of course we owe our written and spoken heritage to the Anglo Saxon roots of our culture, nourished in the mother country which many of us still hold close to our hearts.
P.S. In the United States we don't write colour, we write color and pronounce it just as it's written, that's the case with many words that in English English end in our, usually not the case here.