The colour will be a bit wrong. The taste hardly at all (I thought Dubbel required dark candi though?).
For light Candi sugar just substitute granulated sugar. Belgian sugar is made from sugar beet, whereas UK sugar is made from sugar cane but there won't be any difference in flavour for the highly refined light versions. If you're in the camp that believes sucrose causes odd flavours because the yeast needs to secrete enzymes to invert it you can invert it yourself, see below. Golden Syrup also works as a substitute for light candi.
I wouldn't use Muscovado in lieu of dark candi because dark cane sugar contains molasses, dark Candi sugar made from beets is just dark because of caramelisation and maillard reactions. This is the difference between British (Ragus) brewing sugars and Belgian. No impurities in beet sugar because of the bad tastes.
Follow guide for making brewers No. 1 to make light candi substitute
Or the easier (but not wholly correct) way