How to set different days colors in calendar depend from availability ?

This feature possible to configure, only in the Booking Calendar Business Large or higher versions. Because only in these versions possible to set capacity per calendar, and to have different availability per days depend from number of bookings. Please check more about this here:

1) Firstly please check this instruction how possible to configure your calendar skin by modification CSS file:

2) Then you need to activate this Calendar Skin: "Multidays" at the Booking Settings General page in Calendar section.

This calendar skin have special CSS classes for ability to define different colors in calendar depend from capacity of booking resource (calendar) and number of bookings per specific date(s) (in other words based on availability on specific date). Probably you will be need to copy this calendar skin to separate folder, as explained in above instruction for future customization.

3) Each calendar day cells (in this calendar skin) have specific CSS classes like:

The number at the end of this class is explain the availability per specific days.

So in case (if you have capacity per booking resource as 10) you can configure in your calendar skin these colors for the different color for the different availability:
You can just add this code to the bottom of that calendar skin.

/* Partially availability: 1-7 */
.datepick-inline .reserved_days_count1,
.datepick-inline .reserved_days_count2,
.datepick-inline .reserved_days_count3,
.datepick-inline .reserved_days_count4,
.datepick-inline .reserved_days_count5,
.datepick-inline .reserved_days_count6,
.datepick-inline .reserved_days_count7{
    background-color: #eb5;
     color: #eeeeee;
     text-shadow: 0px -1px 0px #888888;
/* Availability 8-10 */
.datepick-inline .reserved_days_count8,
.datepick-inline .reserved_days_count9,
.datepick-inline .reserved_days_count10 {
    background-color: #1A5;
     color: #eeeeee;
     text-shadow: 0px -1px 0px #888888;
/* Approved */
.block_hints .date_approved.block_check_in_out,
.block_hints .block_booked,
.datepick-inline .date_approved,
td.timespartly.check_in_time.check_out_time.check_in_time_date2approve.check_out_time_date_approved div.check-in-div,
td.timespartly.check_in_time.check_out_time.check_out_time_date2approve.check_in_time_date_approved div.check-out-div {
    background-color: #ee5933;
/* Pending */
.block_hints .date2approve.block_check_in_out,
.block_hints .block_pending,
.datepick-inline .date2approve,
td.timespartly.check_in_time.check_out_time.check_in_time_date_approved.check_out_time_date2approve div.check-in-div,
td.timespartly.check_in_time.check_out_time.check_out_time_date_approved.check_in_time_date2approve div.check-out-div {
  background-color: #ee5933;

P.S. Please do not forget to clear browser cache before testing these changes.