This is a leave-in serum, so it is meant to stay in the hair rather than be rinsed out. The usual moment is right after washing: gently towel-dry your hair until it is damp but not dripping, then take a small amount of serum in your palms. Rub your hands together briefly, then work the product through the mid-lengths and ends — the oldest, most damaged parts of coloured hair — and avoid rubbing it into the roots unless your hair is very dry and you know it tolerates oils well.
Start with less than you think you need. A small amount is enough for shoulder-length hair in most cases, and you can always add a little more if the ends still feel rough. For very fine hair, use an even smaller amount and concentrate on the last few centimetres. If you apply too much, the hair can look lank before it dries, so it is better to build up gradually.
You can also apply it before heat styling. The product is useful before blow-drying or styling when hair tends to feel coarse or look fluffy. In that case, apply it to damp hair first, let it absorb for a minute or two, then dry and style as usual. The smoothing layer helps the hair glide through the brush and reduces the rough, fluffy finish that coloured hair often gets after washing.
If you use other leave-in products, apply the serum first and give it a moment to absorb before layering anything else, so the products do not just sit on top of each other.