"I was not sure which colour is better, so I bought both a red and a white roses"
Is it correct that there are articles and that the verb is plural?
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Using an article is fine, but the noun has to match the article. "A ____" is singular, but "roses" is plural, so you can't say "a white roses".
If you bought one rose of each color, you could say
I bought a red rose and a white rose
I bought a red and a white rose
although I think the second one is informal and less clear than the first. Using the article for each makes it clearer than using no article:
I bought red and white roses
This makes it unclear whether you mean one of each, or many of each, or even if you mean "many roses, each of which were mixed red and white". Using the plural can be OK if there's enough context to make it clear, though. For example,
I offered her a white rose, a red rose, a pink rose, and a yellow rose. She picked up the red and white roses.
Here, from context we already know that there was only one of each color, so "red and white roses" is clear.