Perspective

Perspective Wikipedia [Web] (FREE)
1 Point Perspective Interactive Lesson [Web] (FREE)
2 Point Perspective [Web] (FREE)
Perspective Detailed discussion [Web] (FREE)
New Perspective System [Book] ($$$)

Color Theory

Color Theory Basics [Web] (FREE)
Color Theory Detailed discussion [Web] (FREE)
Colors on the Web [Web] (FREE)
Color Scheme Generator [Web] (FREE)
Color Scheme Tool [Web] (FREE)
Color Theory for Digital Displays [Web] (FREE)

Light & Shading

Anatomy

Z-Anatomy-Z Blender anatomy app [Software] (FREE) (website down)

Software

There are two main types of image creation, Raster which uses pixels that doesn't scale and Vector which uses shapes and can scale but its time consuming to add a lot of detail.
Krita raster painting app (FREE) [Software Win, mac, Linux] (OSS)
Graphite.rs raster & vector drawing app (FREE) (OSS) [Web PWA]
Draw, Sketching & Modeling in 3D ($$$) [iPad] Export capability.

Hardware

XPPen Display Tablets [Hardware] ($$$)
Wacom [Hardware] ($$$) drawing pen tablet Intuos Medium, bigger isn't better.

Books

Imaginative Realism by James Gurney [Book] ($$$)
Color and Light: A Guide for the Realist Painter by James Gurney [Book] ($$$)
Creative Illustration by Andrew Loomis [Book] ($$$)