COLOR BY TRANSMITTED LIGHT

The colors of transparent objects is determined by colors of light transmitted by them.

A transparent red filter held against White light appears red as it absorbs all colors except Red which is transmitted to the eye.

If a Red filter is held against Green light it will appear black as it absorbs the Green light.

If a Red flower is viewed through a Green filter, it will appear black since the Red light from flower is not transmitted by Green filter.

Thumb Rule: If there is a common color in the incident light and the filter, then the filter will appear to be of common color. Otherwise it will appear black. 

Colors of objects in colored light

The color of an object as seen by us is affected by the color of light in which it is observed.

A Red flower if observed in blue light will appear black since it absorbs the blue light. It will appear black in Green light also. In Red light it will appear Red since Red light is reflected.

A magenta object when viewed in yellow light will appear Red. This is because Yellow light consists of Red and Green lights and a magneta object will absorb Green light but will reflect Red light.

Thus, if there is a common color in the incident light and the object, the object will appear to be of common color. If there is no common color, the  object will appear black.