As a suggestion, the transparent rows are heavily bloated, as you can barely see the difference between the transparent tones. You can turn the transparent columns into a single one, only having a single transparent color from the spectrum.
Additionally, 'simple' or not, I don't think that the lack of spectrum was a good idea. You are lacking 'vital' colors like Orange and Cyan in a big scale.
As a first, it's good enough. Props for being a first and keeping the column sizes constant, though, they are not 9 colors per row, but 8. This can create problems, and you would've had a wider range of variety and ability to add more stuff if you had 9 colors per column.
Additionally I don't think the contrast differences between the tones are too big. If I am to group the 3 lightest colors from green and then the ones from blue, they are fairly similar, I think having them in a bigger contrast would be better.
