VC paper has multiple emulsion layers of different contrast. The high contrast layers are sensitive to bluer light, the low contrast layers are sensitive to green light. When making a print, you use a coloured filter to control the light colour and therefore the contrast of your final print. Because the paper produces varying contrast with exposure colour, you will find that blue items in your scene are rendered with high contrast and green items are rendered with low contrast. Red items will be black because paper is insensitive to red.
Originally Posted by clothesontheline
Graded paper has a single layer of fixed contrast. However it's often only blue-sensitive, which means that both red and green items in the scene will be black, which makes it difficult to get a good image.
You might consider buying a small quantity of green-sensitive X-ray film. It's extremely cheap - see links on LFPF for sources. That would get you more latitude, far better exposure speed (10x), no weird colour/contrast variation and you can process it under safelight. If you use (for example) Ilford PQ Universal developer, it can develop both the film and the paper.
Definitely you can/should test your exposures with strips (or small tiles) of paper. Hence the term "test strip".