for layers/masks:
UFOCentral generates this markup:
org.robofab.fontlab.maskData
RoFo generates this markup:
com.typemytype.robofont.layerData

for marks:
UFOCentral generates this markup:
org.robofab.fontlab.mark
RoFo generates this markup:
com.typemytype.robofont.mark

A quick hack of the UFO replacing those entries crashes RoFo on opening.
(I did not reorder the tags but this should not matter in XML, right?)

You could convert the font lab HSV color to rgba with this tiny script:

import colorsys
f = CurrentFont()

for g in f:
    fontLabMarkColor = g.lib.get("org.robofab.fontlab.maskData")
    
    if fontLabMarkColor is None:
        continue
    
    r, g, b = colorsys.hsv_to_rgb(fontLabMarkColor/256., 1., 1.)
    
    g.mark = (r, g, b, 1)

Updated code below will port the colour:

import colorsys
f = CurrentFont()
for glyph in f:
    fontLabMarkColor = glyph.lib.get("org.robofab.fontlab.mark")
    if fontLabMarkColor is None:
        continue
    r, g, b = colorsys.hsv_to_rgb(fontLabMarkColor/256., 1., 1.)
    glyph.mark = (r, g, b, 1)

I've noticed that running the above script doesn't seem to modify the .vfb file, or FLS doesn't notice it, because I can close my font file without being prompted to Save.

Version 1.3.1b (built 1211021533)
OS 10.7.5

1. Run UFO Central in FontLab to export your font to ufo (make sure export layers is ticked)
2. Open the UFO in Robofont
3. Run the following code in RoboFont to get your lay­er­data drawn:

import colorsys

f = CurrentFont()

MASK_LIB_KEY = "org.robofab.fontlab.maskData"
MARK_LIB_KEY = "org.robofab.fontlab.mark"

def portFLmark(glyph):
    fontLabMarkColor = glyph.lib.get("org.robofab.fontlab.mark")
    if fontLabMarkColor is not None:
        r, g, b = colorsys.hsv_to_rgb(fontLabMarkColor/256., 1., 1.)
        glyph.mark = (r, g, b, 1)

def portFLmaskData(glyph):
    # get pendata
    # filter out any single point contours (anchors)
    instructions = []
    pointStack = []
    lib = glyph.lib
    
    if lib.has_key(MASK_LIB_KEY):
        instructions = lib[MASK_LIB_KEY]
        pen = glyph.getPointPen()
        instructionsDrawPoints(instructions, pen)
        # clear the mask data from the glyph lib
        del lib[MASK_LIB_KEY]

    glyph.update()

def instructionsDrawPoints(instructions, pointPen):
    """draw instructions created by InstructionPointPen"""
    # filter out single point contours (anchors)
    pointStack = []
    for instruction in instructions:
        pointStack.append(instruction)
        meth = instruction["method"]
        if meth == "endPath":
            if len(pointStack) > 3:
                _drawPointStack(pointStack, pointPen)
            pointStack = []
            
def _drawPointStack(stack, pointPen):
    for instruction in stack:
        meth = instruction["method"]
        if meth == "beginPath":
            pointPen.beginPath()
        elif meth == "endPath":
            pointPen.endPath()
        elif meth == "addPoint":
            pt = instruction["pt"]
            smooth = instruction.get("smooth")
            segmentType = instruction.get("segmentType")
            name = instruction.get("name")
            pointPen.addPoint(pt, segmentType, smooth, name)
        elif meth == "addComponent":
            baseGlyphName = instruction["baseGlyphName"]
            transformation = instruction["transformation"]
            pointPen.addComponent(baseGlyphName, transformation)
        else:
            raise NotImplementedError, meth

for glyph in f:
    # port the colour marks
    portFLmark(glyph)
    # create/get the layer for the mask data
    glyph.getLayer("FontLab Mask")
    # flip it so we draw on a clean layer
    glyph.flipLayers("FontLab Mask", "foreground")
    portFLmaskData(glyph)
    # flip back we’re done
    glyph.flipLayers("FontLab Mask", "foreground")

print "Done"

Hope that helps.

Many thanks for kindly sharing your script, which works very well. The Mask layer and its content are back into RoboFont.
Looking at my translated file, I've just realised that the anchors (accents) are gone. I was expecting the anchors to fall into the category of "Glyphs Marks" when exporting to UFO. It seems again that RoboFont can't see or display them. I'm guessing it because if I export to UFO from FLS and then import the very same UFO file, the anchors are there.

Did you manage to get your FLS accent anchors into RoboFont?

All the best,

Mathieu