linear interpolation between colors

color.pal.R

color.pal = function(x, col = c("green", "black", "red"), breaks = c(-5, 0, 5)) {
    
    if(length(col) != length(breaks)) {
		stop("Length of col should be equal to the length of breaks.\n")
	}
	
    # change col represented as strings to RGB space
    col_section = sapply(col, function(x) as.vector(col2rgb(x)))
    col_section = t(col_section)
    
    x[x >= max(breaks)] = max(breaks)
    x[x <= min(breaks)] = min(breaks)
    
    color = character(length(x))
    for(i in 1:length(x)) {
        # NA values, grey color
        if(!is.numeric(x[i])) {
            color[i] = rgb(128, 128, 128, maxColorValue = 255)
            next
        }
        value = x[i]
        
        # find which interval the value belongs to 
        interval = which(breaks >= x[i])[1]
        if(length(interval) == 0) {
            interval = length(interval)
        }
        if(interval == 1) {
            interval = 2
        }
        
        # linear interpolation
        col_num = (value - breaks[interval])*(col_section[interval, ] - col_section[interval - 1, ]) / (breaks[interval] - breaks[interval - 1]) + col_section[interval, ]
        
        col_num = ifelse(col_num > 255, 255, col_num)
        col_num = ifelse(col_num < 0, 0, col_num)
        
        color[i] = rgb(col_num[1], col_num[2], col_num[3], maxColorValue = 255)
    }
    
    return(color)
}