Not true. 16 colors at a time. EGA had a color palette of 64 colors. Depending on the card, precise palette cycling can be used to appear to have all 64 colors available so long as too many different pixels aren't used too close together.
That wasn't really a possibility for anything but static fullscreen images, or extremely optimized demos. Even then it would be difficult, since there is no way to generate an interrupt at a specific raster position (unlike CGA, where the pixel clock was synchronized with the system timer chip).
I didn't say it was for full-motion video or arbitrary images. Interactive calibration is possible.