What a cast actually is
A colour cast means the whole image is tinted because the light source did not match the camera's white balance. Indoor tungsten light gives orange, shade gives blue, fluorescent gives green. The fix is to find something that should be neutral grey and make it neutral.
Method 1 — the grey eyedropper
Add a Curves adjustment layer and select the middle (grey) eyedropper, then click something in the photo that should be neutral. Photoshop rebalances the channels so that point becomes grey. Good candidates: concrete, white paper, a grey car panel, the white of an eye.
Struggling to find one? Add a temporary Threshold layer to locate the true blacks and whites, or use a Color Sampler and watch the RGB readouts — a neutral is where R, G and B are close to equal.
Method 2 — Camera Raw
Filter → Camera Raw Filter gives you Temperature and Tint sliders, which are more intuitive than channel curves: Temperature runs blue to yellow, Tint runs green to magenta. On an actual raw file, do this in Camera Raw before opening in Photoshop, where the adjustment is genuinely lossless.
Method 3 — curves by channel
For a cast that varies across the tonal range — blue shadows but neutral highlights, common in shade — adjust individual channels in Curves and shape only the affected part of the line. Global sliders cannot do this, which is why Curves remains the most capable colour tool in the program.
Do not neutralise everything
Warm light at sunset is not a defect. Removing every cast leaves images clinically flat and strips the character of the light. Correct casts that are wrong; keep the ones that are the point of the photograph. Once corrected, deliberate colour choices belong in colour grading.