This code might help you:
defun( DE_setLayerColour (layer, colour))
let((colourNo, colours, n, p)
cond(
(numberp(colour), colourNo = colour)
(!colour, nil)
(t
colours = '(
(White 255 255 255)
(LtBlue 014 210 255)
(Rose 255 121 203)
(Green 000 255 106)
(Purple 166 016 255)
(Teal 121 153 196)
(Red 255 000 000)
(Yellow 255 255 000)
(Blue 000 000 255)
(Aqua 055 247 215)
(Gray 179 179 185)
(Olive 116 196 143)
(Orange 252 199 046)
(Pink 255 178 255)
(Beige 172 138 138)
(Navy 002 168 213)
(Violet 176 000 206)
(Gold 234 190 000)
(Silver 217 224 224)
(Rust 242 127 134)
(Lime 014 249 182)
(Brown 161 004 004)
(Mauve 224 141 202)
(Black 000 000 000)
)
n = listnindex(mapcar(lambda((colDef), lowerCase(car(colDef))), colours), lowerCase(colour))
when(n, colourNo = n + 1, axlColorSet(colourNo, cdr(nth(n, colours))))
)
)
when(colourNo, p = axlLayerGet(layer), p ->color = colourNo, axlSetParam(p))
))