I guess the way around it for veggies would be more vegetables and less pasta. Mushrooms are very meaty and only contain I think, without checking, something like 13 calories per 100g because they are mostly water so yes, doing red day style plans and having quorn fillets, pieces, sausages or mince in place of all your meat would do you really well!
A really really good meal for me on green which helps my weightloss is 3 quorn sausages, butternut squash mash with wilted spinach and steamed shredded spring greens and spring onions in it with a side of mushy peas and lots and lots mushrooms cooked in frylight.
Are you full vegetarian or sometimes are you pescetarian cos you could do a red day and have lots of fish?