Hume and the mind

In his most famous work, A Treatise of Human Nature, David Hume outlined his account of the human mind. Central to this

Hume himself offered up the missing shade of blue as an exception to his account of the mind, which states that all ideas are formed from impressions. The example he claims provides at least one clear occasion where an idea can be formed without the relevant sensory experience.

The problem is this, directly quoted from Hume, Treatise,

"Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with colours of all kinds, except one particular shade of blue, for instance, which it never has been his fortune to meet with. Let all the different shades of that colour, except that single one, be placed before him, descending gradually from the deepest to the lightest; it is plain, that he will perceive a blank, where that shade is wanting, and will be sensible, that there is a greater distance in that place between the contiguous colours than in any other. Now I ask, whether it be possible for him, from his own imagination, to supply this deficiency, and raise up to himself the idea of that particular shade, though it had never been conveyed to him by his senses? I believe there are few but will be of opinion that he can"

I was thinking about this, and it seems to me, that even if the man can form the concept of the missing shade of blue, it is not an example of an innate idea, or an idea not caused by an impression.

It is clear that the spectrum of colour is one that progresses along a set pattern. So each increasing shade of blue is one hue more than the previous. So all the set shades of blue that we can perceive could be represented as such:
BlueA, BlueA+1, BlueA+2, BlueA+3, etc...

So if we agree there is such a set pattern, then by perceiving all the shades of blue, bar one, we have perceived this pattern. And if we have perceived the pattern, (had an impression of it) then we can form an idea of the pattern.

So if we have an idea of the pattern that the shades of blue follow, then upon recollecting the shades we have experienced, we should be able to fill in the gap, that is to have an idea of the missing shade.

However this idea is not uncaused or without a relevant impression. Indeed the idea is a complex one formed from both the impression of the shades of blue we have encountered, and the impression of the pattern. So the idea is not one that is formed without the relevant sensory experience.

The best comparison I can make is to numbers. We may have never directly experienced 611 things, but we can form an idea of what 611 is, because we are familiar with the pattern that numbers follow.

Obviously if we did not have an idea of the pattern we could not fill in the missing shade, but to perceive the other shades of blue, is to perceive the pattern. It may not be strikingly obvious, but upon reflection the nature of the pattern should be revealed.