diff --git a/colors.lua b/colors.lua
index 5eca7a1..0faeb43 100644
--- a/colors.lua
+++ b/colors.lua
@@ -83,7 +83,13 @@ end
 -- Interpolation
 
 colors.interpolate_rgb = function (c1, c2, t)
-	return { c1[1]+(c2[1]-c1[1])*t, c1[2]+(c2[2]-c1[2])*t, c1[3]+(c2[3]-c1[3])*t }
+	assert(type(c1) == 'table' and type(c1[1]) == 'number' and type(c1[2]) == 'number' and type(c1[3]) == 'number' and (type(c1[4]) == 'number' or c1[4] == nil))
+	assert(type(c2) == 'table' and type(c2[1]) == 'number' and type(c2[2]) == 'number' and type(c2[3]) == 'number' and (type(c2[4]) == 'number' or c2[4] == nil))
+	assert(type(t) == 'number')
+	--	
+	local alpha = nil
+	if c1[4] or c2[4] then  alpha = (c1[4] or 255) + ((c2[4] or 255)-(c1[4] or 255))*t  end
+	return { c1[1]+(c2[1]-c1[1])*t, c1[2]+(c2[2]-c1[2])*t, c1[3]+(c2[3]-c1[3])*t, alpha }
 end
 
 local function angle_delta (a1, a2)