Different days color depending from availability per days.

To show different background of days, depending from number of available slots per day possible in the Booking Calendar Business Large or higher version for booking resources (calendars) with specific capacity.

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) The color for pending days you can configure in calendar skin the same as it's done for approved days, so in this case, days will look the same.
And Calendar legend items for pending days you can deactivate at the Booking > Settings General page in section relative to calendar legend items.

4) Now most interesting.
Each calendar day cells (in this calendar skin) have specific CSS class like:


The number at the end of these CSS classes explains the availability per specific day.

In this case you can configure in your calendar skin based on these colors the different color for the different availability.

For example:

/* Partially Available 1-5 */
.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;
/* Available */
.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,
/* P E N D I N G  - BACKGROUND for Check In Approved & Check Out Pending ||  Check Out Approved & Check In Pending    Dates     // FixIn:  */
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;

Disclaimer. Disclaimer. Unfortunately we can not start, right now, some personal customization or custom development, because we have almost no free time.
Check more about, what support we are providing here.
Thank you for understanding.
Please note, if you modify the source code of the Booking Calendar, we will not guarantee the correct work of the plugin and do not support it.