The term “dye release” is used to describe the separation of the plant dye molecule from the plant matter making it available to attach to the keratin in hair. Each plant dye has a specific environment best suited to encourage dye release.
Henna and Cassia
Henna and cassia prefer a slightly acidic environment in order to release dye and preserve the molecules in a state in which they can attach to the hair in the most permanent way possible. Lemon juice is the most recommended acidic liquid, but many people find it to be too harsh for their hair or skin. Other options are apple cider vinegar, orange juice, strong black tea, or even apple juice. Henna and/or cassia mixed with an acid should be left to stand at room temperature 12-24 hours before use. Acid hydrolysis, the process by which the dye is released in acidic mixes, is slow.
Many people prefer to use a neutral mix to release dye when using henna and/or cassia. This has some pros and cons. Neutral mixes are ready to use more quickly. They only need to stand 3-8 hours because enzymatic hydrolysis, the process that occurs in neutral mix dye release, is slower. Less time is needed if mixed with a warm liquid or left to stand in a warm place. More time is needed if mixed or left to stand at room temperature. Using a neutral mix means being able to use different, more mild liquids like water, and including those such as chamomile tea that are great for hair on their own. On the other hand, using a neutral mix does not preserve the dye molecule as well and may not create a bond with hair that is quite as permanent. Henned hair mixed with an acidic recipe will darken more than a neutral mix and stay that way, whereas a neutral mix may not get quite as dark and may eventually fade over a long period of time (months).
Whether to use an acidic mix or a neutral mix is a personal choice, and the pros and cons should be weighed when planning a mix. Either way, if both cassia and henna are being used they can be mixed together, in the same container, and at the same time.
To do a skin test for dye release for henna, put a small dab of paste on your palm (not your arm). Wait 5 minutes and wash off. If you have an orange stain (ideally bright orange) you have very good dye release. If it's a faint orange stain, you may have early dye release and more sitting time is needed - or you may have passed the peak of dye release.
Here is a chart showing how long dye release takes using a cold lemon juice mix at varying room temperatures: www.hennapage.com/henna/how/dyerelease.html
Keep in mind that this method of testing for dye release only works with henna. The chart above is meant only as a guideline. Each henna is unique and may have slightly different dye release timing.
Buxus and Indigo
Buxus and indigo prefer a neutral environment to best release their dye, and they do so quickly. Bottled water is the best choice with using these two plant dyes. It's best not to use any other ingredients (except for a pinch of salt with indigo) as they can be fussy and extra additions to the mix may prevent them from working as well as they should. Both dyes should be mixed with water just minutes before use. Once stirred to a rather smooth, yogurt like consistency, they can be applied or mixed with henna and/or cassia that has already been dye released.