Unluckily this is not possible right now as these predefined colors are taken from a CSS file. You may override the styles be defining your own color-set, e.g.:
/* ===== x-cal-3 ===== */
.x-cal-3,
.x-cal-3-x .ext-cal-evb,
.ext-ie .x-cal-3-ad,
.ext-opera .x-cal-3-ad {
color: #339900 !important;
}
.ext-cal-day-col .x-cal-3,
.ext-dd-drag-proxy .x-cal-3,
.x-cal-3-ad,
.x-cal-3-ad .ext-cal-evm,
.x-cal-3 .ext-cal-picker-icon,
.x-cal-3-x dl,
.x-calendar-palette .x-cal-3 span {
background-color: #339900 !important;
}
.x-cal-3-ad,
.x-cal-3-ad .ext-cal-evm {
border-color: #339900 !important;
}
This would override COLOR_ID 3 to be green instead of dark red.