Medieval people appreciated brightly colored flowers or sunsets or autumn leaves just as much as we do. But they did not have the option of having those colors on their bathroom wall or on their clothing.
The bright colors we take for granted for manufactured items are products of chemical dyes, first developed in the nineteenth century. The Middle Ages had to make do with natural dyes. A few of these could produce very nice colors, but those were luxury items. For most purposes, one had a choice of off-white, muddy green, subdued red, dark brown, dark blue, and maybe unconvincing yellow.
The white was usually off-white, the color of undyed sheep's wool or linen. Linen could be bleached in the sun, and elegant ladies in the stories wore shifts of snow-white linen. Keeping it spotless was an additional challenge.
Black sheep (actually dark brown) produced dark brown wool, which was used for monks' habits (so-called black monks) as well as anything else where you wanted a dark brown/black. Some of the new monastic orders of the twelfth century, such as the Cistercians, went in for white habits instead, because black sheep were rarer than white and their wool was thus more expensive (and "showier").
Purple came from mollusks from the eastern Mediterranean. This so-called Tyrian purple (actually closer to maroon) had been reserved in ancient Rome for colored strips on the togas of Senators. In Byzantium, this purple was so rare that it was supposed to be reserved for the imperial family (hence the expression, "born in the purple" for someone of extremely high birth).
Real red was made from kermes insects, found in Italy. The secret of this vivid red was closely guarded, so that cloth might be sent from the cloth markets of Champagne to Italy to be dyed and come back with its value more than doubled.
You could get a version of dark red from madder, an herbaceous plant with red roots. It worked great to dye your hands red while you were trying to get some color on the cloth. ("No, I don't have blood on my hands!") Indigo, which came from the sap of certain shrubs, could give you a dark blue. You could also get blue from woad, a plant in the mustard family. Yellow was hard, but you could get at least pale yellow from some flowers and especially from pollen (in particular the pollen of crocuses, saffron). You could get green (sort of) by mixing yellow and blue or by embracing grass stains.
Madder, indigo, and woad were all sold commercially, as was saffron, although the latter was very expensive and mostly used as a spice.