How to improve the skin tone after a laser tattoo removal? (Photo)

Thanks for your question.
From the photo, it seems that there is a lot of hypopigmentation as a result of laser tattoo removal.  This is to be expected, as the laser does not exclusively remove tattoo pigment, but can also cause lightening (usually temporary) of the underlying skin, which often improves with repeated sun exposure.
Both the Fraxel and excimer lasers can be used to improve the appearance of hypopigmentation. A series of Fraxel laser treatments and multiple excimer laser treatments are needed. In your case, I would recommend the Fraxel laser, as it appears from the photo that there is also some atrophy/thinning of the underlying skin, which the Fraxel laser can also improve.
It is unclear from the photo as to whether or not there is still some tattoo pigment that has faded, but not completely resolved, which may require further laser treatments to eliminate any residual tattoo pigment.  Often for treatment resistant tattoos, the Fraxel laser is combined with laser tattoo removal.

It can occur due to melanin damage.  After a few sun exposure it should regain pigmentation.  It is very common occurrence.  In reviewing your photo there are areas of hyperpigmentation too.  The pigmentation can be treated in various ways if it persists.  Often after a few weeks pigmentation resolves.  If it persists on can use enlighten or IPL to treat it depending on the skin type.