“Natural wet paint” is a little ambiguous; the mixing of CMYK as suggested won’t work because you’re still adding colors.
If you want results like in Photoshop (as Jon B checked) you need to use L*a*b* space. Formulas for converting RGB to/from Lab and a description is here.
Lab space was specifically designed so that linear changes correspond to what the human eye perceives as a certain amount of color change. This is important because e.g. we are more sensitive to green than other colors, because we perceive changes differently depending both on hue and lightness, etc..
Trying any other methods currently being suggested will not only result in colors you don’t want, but also won’t represent a “constant-looking” change in color, especially if you use this for something where constant-change matters like a gradient.