Ultimate member default blue color #3ba1da and hover color #44b0ec can be overwritten using CSS.

Since WordPress 4.7, users can add custom CSS directly from WordPress admin area Appearance >> Customize >> Additional CSS. Some premium themes provide Custom CSS section under the themes options. You can also use custom CSS plugins which allows you to store your custom CSS independent from your theme.

Here is the sample CSS which you can add to your custom CSS section and replace blue #3ba1da and hover #44b0ec colors with any color you need according to the design of your website.

/* Link colors */
.um a.um-link,
.um .um-tip:hover,
.um .um-field-radio.active:not(.um-field-radio-state-disabled) i,
.um .um-field-checkbox.active:not(.um-field-radio-state-disabled) i,
.um .um-member-name a:hover,
.um .um-member-more a:hover,
.um .um-member-less a:hover,
.um .um-members-pagi a:hover,
.um .um-cover-add:hover,
.um .um-profile-subnav a.active,
.um .um-item-meta a,
.um-account-name a:hover,
.um-account-nav a.current,
.um-account-side li a.current span.um-account-icon,
.um-account-side li a.current:hover span.um-account-icon,
.um-dropdown li a:hover,
color: #3ba1da !important;

.um a.um-link:hover,
.um a.um-link-hvr:hover {
color: #44b0ec !important;

/* Button colors */
.um .um-field-group-head,
.um .um-members-pagi span.current,
.um .um-members-pagi span.current:hover,
.um .um-profile-nav-item.active a,
.um .um-profile-nav-item.active a:hover,
div.uimob800 .um-account-side li a.current,
div.uimob800 .um-account-side li a.current:hover,
.um .um-button,
.um a.um-button,
.um a.um-button.um-disabled:hover,
.um a.um-button.um-disabled:focus,
.um a.um-button.um-disabled:active,
.um input[type=submit].um-button,
.um input[type=submit].um-button:focus,
.um input[type=submit]:disabled:hover
background: #3ba1da !important;

.um .um-field-group-head:hover,
.picker–focused .picker__day–highlighted,
.picker–focused .picker__list-item–highlighted,
.picker–focused .picker__list-item–selected,
.um .um-button:hover,
.um a.um-button:hover,
.um input[type=submit].um-button:hover{
background: #44b0ec !important;

/* Button alt colors */
.um .um-button.um-alt,
.um input[type=submit].um-button.um-alt {
background: #eeeeee !important;

.um .um-button.um-alt:hover,
.um input[type=submit].um-button.um-alt:hover {
background: #e5e5e5 !important;