"Will you hand me a pencil" doesn't mean I want you to hand me at least 1 pencil but possibly 5.
I see where you are trying to come from with that angle though. You can feel free to apply different semantics to "a ___ "but I feel the general consensus is that it is equivalent to "one ____".
Note that "a colored shirt" is not the same as "a shirt with a color". As "A colored shirt" could be a shirt with multiple colors due to the ambiguity of "colored".
E:
From the article:
>In one version of the task, one subject (always one—he spurned testing subjects in groups) is presented with four cards lying flat on a table, each with a single-digit number on one face and one of two colors on the other.
It specifies "one" for the colors.
There's a difference between your examples and the question:
"Give me a shirt." is a request for one shirt.
"You've got a shirt in the wardrobe." is a statement that doesn't tell you how many shirts are in the wardrobe, just that there is one.
Similarly:
"Will you hand me a pencil." is asking for one pencil.
"I have a pencil." is telling you that I have at least one pencil.
Based on en-gb native use.
>So 5 could not have an even number on the back given the rules. //
Having "a number" on one side doesn't mean it only has one. Nor does having a colour on the other side mean it doesn't have more than one colour, nor indeed that it doesn't have colours on both sides.
It's like me saying "I have a coin in my pocket" it only tells you the number is >= 1 assuming I'm being truthful.
If you expect a precise answer you need to ask the question in a precise manner.