Here are a few more inexpensive ideas:

Rainbow Bath Oil

1/2 c light oil (sunflower, canola)
1/2 c distilled water or rosewater
2 drops blue food color
2 drops red food color
25 drops fragrance (optional)

Use a clear bottle - fancy, corked bottles look nice. Pour the oil, optional fragrance and blue food color into bottle. Shake gently to blend. In a separate container mix water and red food coloring (rosewater can be found in some grocery stores by alcoholic drink mixes.) Pour colored water into bottle with oil. To use shake gently to mix colors. Pour small amount into bath water.

Honey Vanilla Bath

1/2 c sunflower or canola oil
1/4 c honey
1/2 c bubble bath (find something scented lightly or with a scent to compliment the vanilla and honey)
1 T vanilla extract

Mix well and put in airtight bottle. Shake gently before using. Pour 1/4 c under running water.