You could use adjectives such as 'light', 'dark' or 'deep'. You could say it is 'like' another colour and explain how it is different. Or you could describe it as a mixture of other colours. Do you have a colour in mind? Try to describe it and then search for it on Google images :)

You can choose from the primary colors (RED, GREEN or YELLOW, and BLUE) or from the secondary colors (ORANGE, VIOLET or PURPLE, GREEN) to describe something. Not everyone sees colors exactly the same way so you can also add the suffix "ish" to describe a color that is nearest to a primary or secondary color, for example: reddish, purple-ish, yellowish, greenish, etc. Tertiary colors are made by mixing either one primary color with one secondary color, or two secondary colors Yellow + Orange = YELLOW-ORANGE Red + Orange = RED-ORANGE Red + Violet = RED-VIOLET Blue + Violet = BLUE-VIOLET Blue + Green = BLUE-GREEN Yellow + Green = YELLOW-GREEN With tertiary colors,you can also add "ish" to be more specific in describing a color; example: yellow-orange-ish .