Placing coloured cellophane over seedlings is a perennial elementary school science fair project. When done properly, this experiment demonstrates the effects of different coloured lights on plant growth. To replicate this experiment at home, you should avoid darkly coloured or opaque cellophane, as it will block your plant's light source and prevent growth regardless of colour.
If you use clear cellophane, your plant's growth should resemble that of a "control" plant with no cellophane. This is because clear cellophane lets in a full spectrum of light and therefore has no significant effect on plants.
Orange, Yellow and Green Cellophane
Green, orange or yellow cellophane inhibits plant growth because it only lets in green, orange or yellow light, which plants reflect. Green cellophane in particular is harmful to plants, since it tends to be darker than other colours and blocks out more light.
Red, Violet and Blue Cellophane
Red, violet or blue cellophane lets in red, violet or blue light, which plants readily absorb. These colours will accelerate plant growth and encourage flowering, as long as your cellophane isn't too dark.