Converting RGB from color sensor into the Munsell Color System
I have some thoughts on your task. First of all colorimetry is a science on its own. I know people working in this field for decades with equipment costing many millions of €. They all share some basic wise words. One is: "Because we all see colours we think measuring colous is an easy task. The opposite is true. Many of the best colorimetrists are colour blind." One second is: "Without light there is no colour". I will write on this later.

Colorimetry in fact is a complex matter. Between a colored piece of stone and a numerical value normally lie a lot of transformations and convolutions. And inbetween there are also lurking many error sources.

This said, I'm sure it will be possible to produce somewhat acceptable results with your sensor. Provided you do a proper calibration and add a calibratable light source.

Speaking of light source. A piece of stone can only reflect incident light (except some uranium/radium salts, you don't want to deal with). Pigments are basically LTI-systems (aka passive filters). This means they can only reflect wavelengths which are present in the incident light. And the will reflect a well defined and individual proportion of each wavelength regardless the luminous flux of the incident light. This means, if your incident light contains nearly no blue light then the stone or whatever you want to examine will reflect only the fraction of "nearly no light" it typically does reflect of any amount of blue light.

So the remission spectra of all pigments get shaped (multiplied) by the spectrum of the incident light. Your sensor sees only the reflected light and therefore will give totally different results for one and the same object depending on the incident light. Look up the spectra for daylight and incandescent light and think about the results of your sensor.

To make a long story short, you can only yield reproducable measurements if you supply your own light source along with the sensor. This is in fact the way all colour meters work.
The only other way was to measure the full spectrum of the incident light. Reasonable portable spectroradiometers for this purpose cost 5000 €. So it's better to go for a light source :).

As they are rather affordable I recommend using LEDs to make up a light source for examination. But LEDs suffer from several drawbacks. They tend to change their output spectrum with their chip temperature and ageing processes. So you have to supply a proper calibration procedure for your measuring equipment. And you better do the calibration sequence before every measurement. 

You then have to design an apparatus which allows the light from your LED fall onto your samples (stones, sediments or so) but prevents other scattered or direct light to enter your sensor. Otherwise all measurements will be crap.

While this is not a direct answer to your question, I think the topic is important enough to write an elaborate pamphlet.

Update

Now some more words more closely related to your problem.

In general loading a huge excel table into your arduino is not necessarily a good idea, even if a big "if"-code array was possible. Storage is scarce on this platform. 

If you combine a lookup table with interpolation, you can save much space. You would have to omit 90% of all datapoints and estimate all in between when measuring. Still the table will be rather big, because you have three degrees of freedom.

Another drawback of big tables and LUT with interpolation is, that it makes calibration difficult. Normally, when you calibrate your sensor, you have to adjust the parameters along the calculation. If you rely on a table, you have to exchange the whole table, regardless if it is something like that excel thingy or a condensed lookup table.

If you go for an analytical solution, as suggested in one comment, the number of parameters you have to update after calibration reduces drastically. Effectively you have to adjust some factors and offsets in the equations for this problem. Of course you will have to learn the ropes of the photometric equations, but you will  have to do that anyway, I think.

Some more words on the design of your appartus. Your idea of a cylinder in front of your probe is good. I recommend to fit it with a rim of black foam rubber, so you can seal light coming from outside. And you have to paint the inside of your cylinder as black as you can. See the interiour of a camera objective for reference.

If it is white you will have multiple reflections and multiple filtering which deteriorates your measurement results vastly.