I'm making Palak dal which calls for yellow lentils-- no luck finding any! I bought normal lentils instead.. Should I expect a big taste diff

I agree with the above two answers and would only add that you might want to bump up the amount of spices you're using, too. Yellow lentils are a little more delicately flavored so you made need a bit more oomph to get the flavor to come out over the earthy green lentils.

Your profile doesn't say where you live, but you might find the right lentils even in a supermarket. I'm in a small town in New Hampshire, and I have seen them in the "International Food" section of supermarkets here. Usually they are with Indian or Asian foods, might be called Dal, or mung dal. They are split and the skin is removed. Similar are red lentils which you could substitute. Probably the lentils you bought will be delicious in the recipe you found -- since they are probably whole with skins on, check the cooking time to be sure they are cooked through (just put one in your mouth -- should not have any crunch). Another substitute would be yellow split peas, but in my experience they take a long time to cook.

It should be fine. the yellow lentils are milder. The green ones have a more earth taste.
given we're in fall cold weather, the green ones could be a superior for the season.