Paint Color Mixing Explanation Needed

I have been physically mixing green and red paints and, rather unexpectedly, the result is coming out closer to blue. I’d like a clear, practical explanation of why this is happening and how pigment behaviour differs from what basic colour wheels predict.

What I need from you:
• A concise, lay-friendly write-up (no more than two pages) that covers the underlying colour theory, the chemistry of common paint pigments, and how subtractive mixing can lead to surprising outcomes like the one I’m seeing.
• Simple recommendations on how to adjust my palette or choice of pigments so I can reliably hit the hues I’m aiming for.
• Where relevant, include quick reference swatches or small illustrative diagrams to make the concepts easy to follow.

This is strictly about physical paints (not digital colours), so please draw on real-world pigment knowledge—artist-grade acrylics or oils are fine examples. I’d like the explanation delivered as a PDF or well-formatted Google Doc that I can print and keep in my studio.